Here's the official Steam hardware survey (pc only):
http://store.steampowered.com/hwsurvey?platform=pc
Win XP 32bit is used by 15.51% of pc users running Steam. Win XP64 by 0.52%. And percentages are dropping every month...
Imo the only rational reason to support dx9 in CoD is some sort of contractual obligation. Nowadays there are plenty of A games that will not run on Win XP. I liked Windows XP but those days are long gone.