Unless I overlooked it, no one has mentioned the reason that is most important for me in wanting to keep as little as possible on C: apart from actual system files.
I upgrade and rebuild my system from time to time, which usually involves a clean install. For example, every couple of years I might get a new processor and motherboard, and you generally can't just stick your old system drive into the new machine and expect things to work right. (Some backup software promises "hardware independent" restores, but I haven't tried that.)
So an OS installation is needed, which generally means formatting C: and losing everything on it. If you've got scads of sample libraries and the like on C:, they're going to disappear, and you're going to have to put them back by re-installing the libraries, which can be painfully time-consuming.
I would much rather keep track of any symbolic links I've made and restore those after a fresh OS install than have to reload libraries.