In another comment I mention the Geograph project which uses cc-by-sa. For some more background to that, I wrote about the reasons for selecting that licence to ensure long-term freedom of the archive here: http://www.geograph.org.uk/help/freedom
What the comment you replied to point out is that if I release something under the GPL and then assign copyright to you, you can legally decide to relicense that code under a proprietary license so that the people buying it from me don't have to abide by the GPL.
If I don't assign copyright, on the other hand, any such third parties would need my permission too.
I wonder if it would stand a chance as a private members bill....
A project that does not require central copyright assignment and that receives sufficient outside contributions cannot have that happen, provided it's licensed under something like the (L)GPL.
Copyright paralysis can be a feature rather than a bug.
Then there are "moral rights" which complicate things in some countries.