do people need to care about copyright assignment? take openoffice as an example. Sun hasd very strict rules about what got into the official OOo tree. Various people wrote patches that sun did not want. they lived outside in go-oo. once this reached a critical mass it was able to split into libreoffice.
thats slightly oversimplified, but the group hurt most by the copyright assignment was sun/oracle. the opensource community just moved on.
now suppose company X has written some nice opensource but copyright assignment requiring program (perhaps a flashy DE, or boot manager). want to contribute without signing the agreement. just make a branch, DVCSs make it easy. you get all their patches, they refuse to accept yours. you win.