There's a world of difference between releasing contributions under the same copyright (i.e. if you're contributing to a GPL'd project, release your code under the same GPL license) and assigning copyright to the FSF. I think that's the turn-off for many. It isn't a matter of 'what would you even do with' the contribution, it's a matter of recognition and making it much harder to change the licensing terms down the road.

For those who just want to bury their head in the sand and pretend copyright doesn't exist, they will be the first to complain when the code that they wrote is taken private and commercialized (i.e. look at the licenses this has been an issue for)... making code 'public domain' allows for that.