Why does anyone use adultery as a ground for divorce if it can be so expensive?

That’s a question. Adultery as grounds is not as common as it once was, but some believe that it can be useful in a property settlement negotiation or in a spousal support dispute. Judges still look with disfavor on adultery, and some lawyers believe that proof of adultery may sway a judge to a particular point of view.

Even if the disapproval of a judge confers an advantage, however, some angry spouses, particularly women (hell hath no fury like a women scorned) use adultery as a way of achieving a moral vindication. In this routine, the scorned spouse has the psychological satisfaction of saying, "Yes, the marriage failed, he was an unfaithful s.o.b., and that’s the reason."

The difficulty here is that outside of her family and maybe some friends, moral vindication does not mean much. So many lawyers try to discourage angry spouses from such an approach.

One of the best reasons for not using adultery as a ground is that the allegation means that the divorce goes to trial, and that increases the cost of the whole dissolution dramatically. That’s where uncontested and no-fault divorce come into play, opening the way for a marital dissolution that while not emotionally painless is at least less expensive.

Furthermore, some believe that alleging adultery is scandalous or shameful, and reflects poorly on both marital partners, and so it might not be alleged.
Some people also believe that only people who are dissatisfied at home stray in search of satisfaction away from home.