GPL is a hostage negotiation, or at best barter. I'll give you my code if you give me yours. There is nothing "free" about it in my mind.
MIT/BSD/Apache is true freedom "You can have my code, no strings attached, do what you want with it, just promise you will tell people it is open-source and who wrote it!"
@drewfer LGPL is an improvement but still stifles contribution for much the same reason
@freemo what about LGPL? You can link liberally but have to provide source for improvements.
How ironic is it that the GPL protects and grows the software commons whereas simple attribution doesn't.