Say what you will about WMV, but in reality it's the only codec everyone (read: XP users) has on their computers without downloading anything (after all, that "built in" factor in general is what Microsoft lives on).
If I want something only for myself, I would use XviD (and in the past DivX), but if I want to either send it to someone who isn't a pro or post it in a commercial site of mine, I would without a doubt use WMV.
Well, supposedly MPG is the only other "built in" codec (and even more so if you count none XP users), but it's like a WAV file compared to MP3.
And that's before even considering only WMV, Quicktime and RM can use streaming servers (and the latter two are just as annoying as WMV if not more and aren't even "built in")!