I think this was fixed, but have no proof. It was not just a problem in OS 9, but also a problem in the early versions of OS X, which is a large reason why, back in those versions, Apple did not even allow you to store anything on the desktop.
What I tell people now, and what I do myself, is that instead of dumping stuff on the desktop, create a folder on the desktop and dump everything inside that. Apple did help reduce the problem significantly when they designated a Downloads folder a few OS X versions ago, so that downloads would not pile up on the desktop anymore.
QuickTime Kirk... the problem was not as simple as you described. There were issues around the fact that the Desktop is a "special" folder because it also functions as a perpetually displayed window (which has implications for tracking graphics objects, storing them in video memory, updating them, etc) that caused resource problems because of the way Apple was doing it.