Just a quick thought that has passed my mind while taking a shower today.
If I were at Google I would give a few more points to the page that consisted only of valid markup. Notice when I say valid, not semantic as the latter one gives you more google points beyond a shadow of a doubt. Valid markup says that the author simply cared more than the rest and as such it is more probable that its content is more relevant (but that, of course, is not true for all cases, take hosting blog services for example), so giving more points to such page would certainly makes sense.
I may, of course, be wrong