Yeah, I figured as much (as a Microsoft game I bought a long time ago was the first to start the trend), but that still doesn't mean that it makes the least bit of sense.
Well, yea, it does -- if you do regular backups of your data, you only have to backup one folder and its sub-folders to get everything, rather than having to dig around in Program Files to find them all.
Also, remember that XP Home shares the same code base as XP Professional, and Pro's security is such that users can be restricted to writing to their My Docs directory only, and no place else. So Program Files would be locked out to anyone but power users and administrators. And no user can see the contents of any other user's My Docs directory.