I'm assuming fans who are talking about Marble leaving early are thinking he'll make the same degree of jump from his sophomore to junior year that he did from his freshman to sophomore. If he does, then that'd be great, as Iowa will likely be playing in the NCAA Tournament if he gets that good.
My guess, though, is he'll be basically the same his junior and senior years but just really refine his game, including improving his shooting consistently to a point where he's a real threat from the three-point line. If he can do that, and become a consistent defender, I could see him getting an NBA shot. I'd agree that unless he's a first-round lock after his junior season, it would benefit him to play out his senior campaign.
One thing to watch: this will be his first year he won't be asked to play a lot at the point guard. Will he be able to flourish at the small forward or shooting guard position, or will he need more work to fit in to this role playing against players of similar size and athleticism?