Multitudes of estates are GENERALLY insignificant, except: as a 3rd pile; in a rush scenario, where grabbing all of them quickly may overcome some other deficit (though this is hard to do), baron, silk road; most importantly, the first estate over is really important, because it turns a tie into a win (or a loss into a win as p1); but then, given that your opponent has the same opportunity, that makes the second estate important in several cases. And by then, hey, one more estate equalizes out a duchy.
Indeed, I disagree with the statement that most games are decided by a 5-3 province split or a 5-3 duchy split. Most games don't see all the duchies bought out. And probably only a plurality are so dependent on the split of provinces - alternate VP sources become very important, as well as 3-pile or colony endings.