A complete RB is always nice; someone like a Terrell Davis who can do it all, and is dominating to boot. The thing is, Reggie Bush can be (as it has been stated) very much like Marshall Faulk. He can carry the ball 20 times, but you can also play him by just giving him 20 "touches" throughout the game. It is up to DD (unless he gets traded), Morency, and Wells to pick up the slack in the running game.
Simply by having Bush on the field on 3rd and long will make us better at pass protection. Will people even want to blitz for fear of a screen to Bush? How much worse will it be when we use a double screen, fake it to Bush and throw a TE screen to the other side? Having a Marshall Faulk-like player creates so many mismatches and problems that your offense gains the advantage.
If the Texans do end up making the trade for Troy Williamson like we've heard, we could see the second coming of the greatest show on turf in Houston. Bush, Williamson, Mathis, Johnson... that is some amazing speed. If the O-line gets fixed and we get a solid DC, the Texans will be a much more exciting version of the Colts.