I'm not a fan. The DM already gets so much of the spotlight, he shouldn't need any more by inserting himself into the party. I'm flexible on transient allies, as a player it can draw me into the game more when most of the other PCs know what I do, the ally can provide someone outside the party to bond with and provide some exposition without too much. But when you're several sessions into the game and every few combat encounters are determined by a DM controlled character, PC or not, you don't feel very heroic.
Sometimes there are involuntary exceptions, I've had a group that did need some healing power, so they kept the Cleric that was only meant to assist them for their first adventure (these players tried to recruit every NPC they came across really, and tried to get every one of them to aid in combat regardless of use, except for the guy who they found out later was a deity traveling with them for a brief time, they of course made sure he stayed out of the way of those vicious level 4 cultists, the young farmhand with no combat experience however...). The Cleric did give the advantage of being at his temple while the party planned, so when we got back together the next week it gave them a reason to go back over it when he didn't know what was going on as a nice refresher, but I did have to write real stats for him, bleh.