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.
For example, one possible reason to sign a CLA is if you want to contribute, and the repository owner will not allow you to do so otherwise. It may be that you value contributing more highly than what license later versions of the code use. At least for me, that would typically be the case.
But really, licensing things under GPLvX-or-later is the best way of handling licensing updates with many contributors.