Two other good reasons to run XP on a Mac:
1) You want to switch to MacOS X, but really don't want to leave behind Windows. You can try MacOS X, rebooting into XP as necessary, and still have a useful computer whichever way you want to go. (Which can also be fairly nice looking).
2) You're a family, and you want to use MacOS X for everything, but there's the occaisional bit of work you have to do on XP. No reason to have both computers around.
Other reasons that may work:
1) Macbook Pros look nicer than the laptops that the office buys (too big/too heavy/too bulky/whatever)
2) Office has people that really work better with OS X (each has its strength), but the ability to run XP on them is handy in case a Mac has to be quickly repurposed. Or to run that one vital program that just doesn't run on MacOS X.
3) Run a computer lab, and instead of having 3 of them (for Mac, XP and Linux/Unix), equip one lab with all Macs, and boot between them all. One set of equipment (same model/build/configuration), one master system image, and everyone can do their work. (Rather than two distinct configurations, excess equipment outlay, etc).
And of course, the obvious reason...
GAMES!
Face it, Windows has a lot of games (a large amount of crap, but that's to be expected). If you have a nice Mac to do your real work, you can then kick back at the end of the day and game on the same machine. Because as much as Mac users hate to admit it, sometimes it's fun to play the same games rather than wait for a port. Sure they aren't killer gaming machines, but if you play console games mostly, the occaisonal PC game isn't going to hurt.
Edit: Forgot the most obvious.