I once signed an employee release agreement. I wasn't really being let go, so much as the store was folding, but the company still existed so they needed us to all sign.
I've heard that this same company went completely under in the last couple of years and when the new owners talked to their staff they insisted that they all sign a release agreement form that said that they were not going to get any of the money owed from the previous owners.
I think most employees signed, because they desperately needed the jobs. Unfortunately, often people need the job so badly they will sign anything.
I'm really glad I'm not working for that company anymore.