in defense of "contributor agreements" or whatever they are called nowadays

I think its entirely ethical for a central authority, even a for-profit one, to make that sort of request as long as they contributor really does have the ability to say no. As long as the incorporation of contribution in question is not held up because of a lack of requested assignment, then I don't see a problem with.

I have no problem with contributors choosing to gift their copyrights as long as its a gift freely given with no explicit or implicit strings attached with regard to their standing as a contributor or the incorporation of their submitted work.

-jef