Chris -
I think the "Krenov" style planes usually use shorter blades because that's what Krenov used. His opinion, at least from his books, was that he preferred the low slung style, particularly because it it allowed a variety of grips in different situations, whereas a traditional style plane sort of forced you into a couple of simple grips. I'm not sure there's anything wrong with the grips a traditional plane presents, but if being able to hold the plane in a variety of different ways is important to you, a shorter iron and a low slung body makes it easier to get there. Certainly, if for whatever reason you want the "low slung" style Krenov appeared to like, a longer iron could dig into your hand in certain grips.
Does anyone sell new tapered irons, or is the best source for those either making/tapering yourself or picking up a used vintage iron?