I don't think Rolen on the roster impacts Heisey as the back-up in CF. But having Rolen/Hannahan as the back-up with Frazier moving to the OF is a better option should Ludwick or Bruce go down. If everybody stays healthy, the difference between having Rolen or Paul on the bench probably doesn't move the needle that much as far as how many games the Reds win. The Red's simply need to be able to deal with injury. We only have to look at last season when Votto was out for an extended period to see that having both Frazier and Rolen on the roster largely helped the Reds to win 97 games in spite of losing the best hitter in baseball for an extended period.
A Rolen/Hannahan platoon would probably be a good enough to start and be relatively productive in the 6 hole on a lot of team's in baseball. It woud add a depth element that would help the Reds withstand a long term loss to one of the middle of the order bats that simply doesn't exist now. Remember, a long term loss of Frazier or Votto means the RH PAs at 3B will go to Jason Donald (or worse yet, Hannahan will be in there against LHP). I don't see that same scenario in the OF with Paul. Even if Rolen retires and one of the OF goes, down, I just don't see Paul being in the mix for increased PT. Heisey or maybe Hamilton will play CF and Choo will move to a corner. Personally, I think that would leave the middle of the order a little light and Hamilton needs to prove himself at AAA before he's penciled into the plan. Heisey is going to get 250 to 300 PAs spotting in as it is now. I don't really want to see him getting another 125 or so PAs if one of the corner guys goes down and I certainly don't want those PAs going to Donald.
For me, I want Rolen because its pretty simple. Everybody stays healthy, it doesn't really matter. One of the corner guys goes down, Rolen/Hannahan is the best insurance policy.