I'm playing devil's advocate here pretty much, but I'm being serious.
If you want to use my stuff, so you are the one that is going to be signing my contract and agreeing to my terms and conditions, not the other way around. That's the way it should work, and expecting anything else is ridiculous. If I want to reserve the right to retract anything, I should be able to (so as long as it's stated). Of course, I'm not saying that I should be able to randomly change my mind and go bat**** calling people thieves for using my stuff because I suddenly decide I want to, but the people demanding documents to protect their things it should be the authors not the people asking them.
I actually hold that "give credit, don't claim it as your own, don't try and sell/make money off of it (without asking for permission), and you're golden" rule should be the default unwritten contract, and you don't need a signed version of that.