This is a great post and very true- however, in terms of predicting who will pan out to be GREAT, I'd say many (if not most) American distance studs have had and do have silky smooth form. Centro, Andrews (even though his upper body may not look as smooth as Centro, his legs/hips are perfect).
Jager, Murphy, Solomon (he has faded, but he burned bright). All of these guys looked like athletes and studs in the making. I'm not saying Hunter can't be that guy, but he is certainly in a different mold.
And for the record, I'd say that Centro, Jager and Murphy are the only true studs (outside of Rupp, obviously) that the US has, and they all have and always had smooth, smooth form. Not a necessity, but surely a useful observation.
(And then there is Nick Symmonds, who ruins any argument for having nice form).