This depends very much on the position of that bit of userspace in the dependency chain, for me.
Upgrade glibc? Much more hair-raising and rarely done than a kernel upgrade. Upgrade some heavily-used library like libpng to an incompatible release? Not common, and not often done. Upgrade a userspace performance counter library? This isn't going to be the most widely-used thing on earth, upgrading it should be easy. Plus, as deater points out, random users can do this and keep it out of the way of other users completely. Random users cannot upgrade kernels, no matter what they do. Only the machine's sysadmins can do that, and often refuse on production systems unless the need is utterly horrifically critical.