Number one reason is that most will not deal with the MS Blaster (and varient) worm that will walk right back onto a fresh XP install even if you have your antivirus running. Typical exchange about such at http://reviews.cnet.com/5208-6142-0.html?forumID=5&threadID=21505&start=0
Number two reason is that many are not going to like learning that Microsoft doesn't install the latest drivers for motherboard, USB, video and more. As well some USB items will cause headaches if you don't unplug them while installing the OS.
For me, dealing with some spyware is the easy fix. My automatic tools are Adaware, Spybot, Cwshredder and for completeness Housecall. If the bug won't go, then a LOG FILE from HIJACKTHIS is produced and if I can't make sense of it, then it's posted for comment.
For Windows NT/2000/XP/2003, you can sidestep issues if you boot the OS CD and use it's tools to partition and let the OS installer format.
Formatting is for (in the Windows OS's) Windows prior to Windows ME. i.e. 1, 2, 3, 95 and 98.