Unlike child support, spousal support is often very difficult (if not impossible) to modify after the divorce is final, so getting it right the first time is a must!
Don't throw the baby out with the bathwater. You can write provisions for relief into a non-modifiable agreement. Mine had a provision affording me a temporarily reduced monthly payment in the event that I lost my job.
The problem with a modifiable agreement is that it leaves couples entangled. Instead of dealing with their problems independently, they will continue to pull each others strings through the courts.
If you feel need a safety net, buy yourself some insurance.