Not really.  It's more for ease in relicensing.  When contributers don't assign copyright, the license can never change, since their contributions bind the original project to the original license.<p>Emacs is a good example of this working.  Because Emacs is copyrighted by GNU and not its contributers, it was a simple matter of changing a few strings to change the license from GPL2 to GPL3.<p>For projects without copyright assignments, this is not possible; the Linux kernel will be GPL2 forever, even if someone wants to change.  Some copyright holders are long dead, and contacting their estate to change some terms in the license agreement is impractical.  (This is a shame, too; a GPL3 version of Linux would be great for hardware hackers everywhere.)