Seems like it's simply a situation of numbers.
3 third basemen. Why? All righthanded.
1 lefthanded bat off the bench - Paul. Have to have it.
I don't see any logical room for him. Sure, you want to mess up the typical depth, then bring him back. But Frazier has been given the starting position, so it's simply a matter of the bench.
8 position starters
C - Mez
3B - Hannahan
SS - Donald
CF - Heisey
OF/LH bat - Paul
With all the concern about Choo and Bruce manning CF, Heisey is the best backup option there.
Paul is the LH bat off the bench.
Hannahan is making pretty good money for a backup.
Rolen doesn't play SS, so it's Donald.
I don't see how it happens.
I agree with Kal Daniels. Gentleman's agreement that if Hannahan or Frazier get injured, Rolen gets signed. He stays in shape and rests his body until needed.
And then there's the money. Play for the Reds for cheap or go to L.A. for good money?