Only under the assumption that Vista has something to offer that couldn't be put into XP. Invalid assumption, of course.
It's not just about anything that Vista gives, but from Microsoft's point of view it's about support. It would cost a lot of money to put equivalent features in both XP and Vista. You saw they didn't want to do IE7 for XP until people got pissed about it. It also costs Microsoft money to continue to support XP for much longer, and ending OEM sales is the first step towards that end.
In any case, XP will be available until January 31, 2009.