There's two problems with that. The first being, as I mentioned, he's kind of absent for a decent chunk of the game - think I'm about to reunite with him, but he's been gone for quite a while now. Second, his one real tanking ability thus far (besides generally being much more durable than the others) just has him take some of the damage for other characters when they get hit, it doesn't actually draw enemy attention or the like. Granted, switching direct control to him would draw enemy attention to him since the AI is designed to mostly focus on the player controlled character, but their attention would just go right back to the others shortly after I switched back. And, as I mentioned, he's kind of the least fun character to play, so I don't really want to spend much time actually playing him.
You could play that way, sure, but I think that illustrates precisely the problem I was criticizing with the way things are set up currently - that sounds less like a fun, intentionally-designed way to play an action RPG than it does a method of abusing the way this one's AI is designed.