i don't think it can be called unnecessary when windows is in question, but it's certainly true that XP is pretty stable and that lots of people use it on stage.
you can use a separate install of xp for audio/performance as well. strip out all the window$ cruft. see
http://www.musicxp.net/ (this is good in general)
and
http://www.blackviper.com/WinXP/supertweaks.htm (more hardcore)
you can also make XP run under a custom shell for any applicable app. see:
http://www.ableton.com/forum/viewtopic.php?t=9809
of course this was written specifically for Live, but it should apply to anything. keep in mind it could effect audio card drivers.
all that being said, yep, i think it's still safe to say the mac will be more reliable, but that's because it's what i have at the moment. ;) windows machines are used on stage all the time tho so don't sweat it... you just have to be a little more aware of the system issues and give the machine more TLC than you would a mac.