Re: 5/29/13 Reds v. Indians - same teams, different ballpark
This is to give Hannahan some at bats, particularly against his former team in his former ballpark.
Dusty said last night that he wanted to give Hannahan two games in a row to give him some playing time.
If he had an obvious DH choice, he would have used him. He doesn't so Dusty is using the opportunity to use Hannahan, a good backup player who hasn't had much time this year.
I agree with Redread on why Frazier is playing third and Hannahan DH, rather than switching them.
The much more interesting question is who Dusty uses tomorrow against a lefty, since his only righty bats on the bench will be Izturis (switch hitter) and Mesoraco (backup catcher).