I'll answer for my husband:
1. He spent all but his working hours with the kids before separation, and with him, it was important interaction, like cooking, chores, cleaning, homework. It was not just sitting around & watching TV with them. He and his ex both work full time, but somehow she is convinced that the time SHE spent with them was more valuable and therefore she was the more appropriate parent. She is the one who pushes the sports and pageants and arranges those schedules and arranges to transport the kids to thier special events. He was the one who was left to parent whichever two were not at a special event at any given moment. If she was at home and there was no special event, she refused to lift a finger. She had some thing like if she was going to work, she was not going to cook or clean and he could either do it himself or hire someone to do it. As much as she spent, they could not afford to hire out for cooking, cleaning, laundry, yard work, so he did it all. When you figure it out, there are 168 hours in a week. They each worked 40 hours and had 2 hours daily commuting (her commute was longer, but she also ducks out of work early on a regular basis), so they each had about 120 hours per week with the kids. He is now reduced to spending an average of 40 hours per week with them while she has an average of 95 hours per week with them (there is some overlap because of all the activities they're involved with that he gets to go to and see them, though technically they have been given the impression by her that they're not to talk to him at football games if they see him, if it's her day with them and not his. , which is ridiculous and embarassing to them, but what can you do).

He has been reduced to a part time special event parent when he used to be the one who did all the business of daily life, instruction about how to be a good citizen, a contributing member of a family, etc...

2. For being the primary parent, she now has the duty of teaching them about how to be a good citizen, do their homework, balance their lives, be a contributing member of a family. And she is failing miserably at this. The kids are showing scary signs of having zero background in this... Their mother makes fun of their father for wanting to help them do homework, and chides him for wanting them to have a schedule to do it. They pick up on this and run with it, because they're teens and they'll run with any little bit of freedom you give them... and so they end up mostly failing till the end of a semester when it's a panic to get enough homework and make-up work and extra credit to bring the grades up enough to pass. One of the kids failed often enough under her primary residential responsibility that he is now going to be unable to get admitted to 90% of the colleges he's talked about going to, because does not have enough credits in a particular area and not good enough test scores (she did not think it appropriate to help him learn how to take the SATs or even convince him to get a practice book just to see what it was all about before he walked in to take it)... it's just sad. My husband was afraid this would happen when he first walked, out, but didn't realize that his act of allowing her to keep the kids 24-7 and keep control over their schedules for the first several months would result in a status quo that would make this a permanent solution in the eyes of the courts. He KNEW that long term exposure to her priorities without significant exposure to a different set of priorities would be a detriment to them, but by the time he was settled in a new place and ready to take them, she had started working every dirty trick in the book to keep them from him and as a result he ended up with a parenting schedule that would have been "traditional" if she had been a stay at home mom & he a disinterested work-a-holic. And by the time a year had passed to where the judge would consider a change, the kids were old enough taht a change would be traumatic... we've only got a few years left till all are out of high school.

The principle behind this point is that these kids had two parents and deserve to have a full share of each parent's manner of handling life. It's expected that each will disapprove of the other's methods & priorities... after all, they're getting divorced... but the kids still deserve to have as much time as possible with each parent. And if that parent truly is neglectful, strange, or whatever it is that bothers the otehr aprent about them, well... the kids can figure that out and judge for themselves as they mature and learn about the rest of the world. They will at least have a CHOICE in the matter, plenty of time exposed to each, apart from the other, so that they have a reasonable basis for MAKING a choice. It will be THIER choice and not one that's influenced by the other parent.

3. He only wants 50% of the time with the kids. The principles he's talked to me about, about how he deserves to have a fair share of the kids' time and they deserve a fair share of his time, they deserve a full, two parent family even if those parents are separated... it applies the other way around as well. If either of them was a physical danger to the kids, it would be different, but it is clear that neither is... if there are long term, unsupervised visitations permitted, then there is no credible showing that there is a danger to the kids, so if either parent WANTS more time than they already have (up to 50%) it's fair to give it to them. And it's sad for the kids if the parent does not want that.

I know some mothers who are disappointed that thier own exes do not want 50% time with the kids. THIER reasons have to do with career needs, now that they're split and need to have a fair amount of time to devote to their own career development, it's not fair that they're saddled with 80% of the kids' time and therefore they're always the one who has to take the hit if the kid is sick or whatever. Or they talk about how they can't move on in their personal lives if their ex never takes the kids or is sporadic about it. It's tough to have those first several dates if you never have any reliable days when you are without children... those first several dates where you should never consider getting the kids involved (unless the kids were the matchmakers in which case you can't really help the fact that they KNOW about the dates)...

Now for my OWn observations:

I also know several people who were madly in hate with their exes and furious about the fact that they were separating, who were convinced to try the equal parenting time thing... 2 of the women had little babies (one had 18 month old twins at the time of separation, one had 5 kids, ages 3 months, 2.5 years, 5 years, 9 years & 12 years)... and now, years later, they're all very happy with the custody arrangements. They're not all thrilled with the financial arrangements, but they're all doing quite well with the equal parenting time thing.

One of my friends has actually become a parenting coordinator out of this (she just got her degree at the time of the separation and working through their issues really gave her incentive to try that way of going). Another has really climbed the ladder of her title company... the third friend is actually the father of the 5 kids. He's a doctor and built a daycare facility into his office building for the purpose of taking care of the little ones as well as before & aftercare from school. His employees are also welcome to use the daycare room. It's a great setup. His ex has become an R.N. and would have never had that opportunity if she'd had to stay the primary caretaker of 5 little ones (3 of whom were pre-schoolers). Truly, in every one of these situations, the kids are thriving. They don't see it as weird, awkward or a pain in teh neck to have two houses. They see it as no different than I saw my grandparents' house when I was growing up. Mom & Dad would drop us off there and go on vacation from time to time on thier own and we'd have fun with Grandma on our week with her, we'd have our own rooms in grandma's house and wouldn't skip a beat in settling in. It was just fine. And these kids who have the 50-50 thing are just fine as well. It's a joy to see this work, expecially when I remember all three of my friends in the support group we've been in, at first, all three were WILDLY in disagreement with the 50-50 schedule (Doc was wanting 100% becasue he believed his wife was neglecting the kids and he'd rather have a nanny than allow her to take care of the kids, is how bad it was)... and I also know how much these parents continue to hate each other. They TRULY can't stand each other. But for the sake of the kids they've figured out how to work together with the 50-50 schedules. It's kind of amazing. The best results I've ever seen in watching the kids growing up and hearing how well they're doing. Yeah, we hear how Doc's ex never combs the kids' hair & they come back from her house all stinky like she never washes them... and we hear how the parenting coordinator's ex is always trying to pick the kids up early if he knows his former mother in law is in town and doing the babysitting on their mother's days... he wants to just mess with his former mother-in-law by proving he has the right to have the twins over her.... yeah, there are always stupid little panics like this, but in general the parents keep taht stuff to themselves and deal with it. And the kids... when we hear how the KIDS are doing, it's almost always great. We almost NEVER hear from these kids that they're falling prey to the typical "divorced kids" syndromes.