Donald Brown proved in 2011 to be a more reliable back than the bust he's been labeled. That said, the cupboards are pretty bare behind him as NFL.com's Gregg Rosenthal points out:
It's not like there are great options behind Brown. Delone Carter, Darren Evans and late-round draft pick Vick Ballard are the only other players on the Colts' depth chart.
Joe Flacco, Matt Ryan and Cam Newton are recent rookie quarterbacks who greatly benefited from a strong running game. It's hard to see the Colts doing Andrew Luck the same favor.