XP seems to have moch more effort put in to actually running it on Parallels/Bootcamp. That's the primary reason I chose it to run on my MB. Additionally, since MB's have the integrated graphics, I figured running a heavy graphics app like Vista, in addition to OS X, and everything else I usually run, was just asking for trouble. Besides, I only run XP every 2 weeks to fill in my employer's timesheet application which is a proprietary Windows-only application.
-Dave