I think this kind of gets to the heart of it--options.
When Bell returns the playbook expands as Payton has more options to throw into the mix.
My best guess however, is that we'll see a lot of Bell and Bush together in the lineup at the same time. I tried to keep track of what went down when Pierre Thomas was in and I did not see a lot of Reggie on those plays. I think it will be more likely that Bush will be in at the same time as Bell, in much the same way as we saw Reggie and Deuce work together in 2006.