At the moment Internet Explorer 9 is the only browser available (ignoring the apps which just wrap IE up).
Think about when an updated IE ships. Do we really want to make everyone choose a specific version? I would imagine most users aren't going to even know.
As far as I know Windows Phone 7 Mango bring IE9 to the fight. But older than this use the IE7 (or IE7 Compact).
In next version probalby we will have another version of browser capable of have css transitions and transforms like IE10 Platform Preview.
Som We need the versioning. I know there aren't so much people with version prior Mango now a days but with the apollo almost here probably we will need questions about IE9 and IE v.next.