Yes, well thanks to the way PCs love to autostart code from any media that's inserted (how can I turn that off under XP!?) the same risks apply there as on the Macintosh. A funny thing about the WDEF virus, though: the same hook that allowed WDEF to spread also made it possible to make a floppy's windows show up in a different style from normal system windows. Probably not totally practical (especially since any properly-configured system would have a WDEF-squasher installed), but one could still use related (not virus-spreading) techniques for some other interesting effects. One of my favorite was putting wctl resources in applications or desktop files (these change default window coloring). It was at times very nice to be able to have a few key applications' windows easily recognizable by color.
Yes, WDEF was a useful driver back in the day. It could be used for odd things like round window regions, etc.