I will assume you mean keeping for the rest of the season.
I think he is better than having a terrible closer like Politte. Dotel gets k's, era, and whip for you guaranteed. And has gotten a good amount of wins this year too.
Also, if anything were to happen to Wagner, Dotel would suddenly be a first rate closer. This is what really makes him a lot more valuable than say his teammate Lidge. Dotel and Wagner would have to go down before Lidge became a closer.
I think the best middle relievers to have are the ones with potential to improve their situations if injuries or trades occur (i.e. Dotel, Santana).
I happen to have both Dotel and Santana right now which is probably overkill, but I have a huge lead in wins and need the k's, whip and era help. So I can afford the risk for now.