There are valid reasons to sign a CLA, though they are vary rare. I believe the FSF CLA is the only one I would ever feel confident signing (because it very strongly restricts what they can do) -- mainly because it means that the FSF has a much stronger case (legally speaking) if it decides to file a copyright suit against an alleged infringement of the work. From my (not-an-American-and-definitely-not-a-lawyer) understanding, American copyright suits are much stronger if you hold a majority copyright over a work rather than only having a very small set of contributions you own.
But really, licensing things under GPLvX-or-later is the best way of handling licensing updates with many contributors.
But really, licensing things under GPLvX-or-later is the best way of handling licensing updates with many contributors.