Also, if my X-partion crashes badly, which is not unlikely considering how flaky X is, I can just re-install X, knowing that all my apps are safe on another partition, as well as my 9 and my Classic.
Okay, I've heard anti-OSX arguments, but none claimed to infect and destroy the whole partition. If an OS is slow, or crashes (note: mine doesn't crash, and is only slow cuz my comp is old; but these are arguments i've heard) it doesn't mean that the hd is in danger. If my OS totally crapped out, I can just as easily re-install x on the same hard drive (or partition) as always. Nothing to stop me. Re-installing the operating system on the same partition as apps won't affect any programs any more than on a separate partition. Sys files should be kept in a diff part of the hierarchy anyway, so if you want to purge the OS, you could delete the "System" folder (for OS X, or "System Folder" for os 9).