Democrats need every Senate seat we can get, even a horrible right-winger like Joe Manchin. Manchin's only use is as a vote for Dem control of the Senate.
He would be useless as governor. Who cares about who the governor of West Virginia is? It's an insignificant state and it clearly wants Republicans to hold any office that isn't held by Joe Manchin.
I'm gotta make a bold prediction, he's gonna get Blanche Lincolned in 2018 if he tries to run for re-election in that senate seat.
Considering the trend WV is going, it's not exactly that bold. In fact, it's more likely to happen than not in the off chance that he runs.