Lambert obviously didn't approve of Bents attitude. Who's fault that is who knows. One things for sure though you ideally keep your best player on side. Lambert & Villa have lost out as a result
He's not the first manager to have a problem with Bent though. Bent can finish well, but he doesn't add much else. There was a graphic on sky sports recently that showed he had one of the three fewests "touches of the ball per game" stats. It seems accepted by most that he doesn't do a lot of work he just gets on the end of things. If you're struggling to get the ball out of defence/midfield, you need a forward who can do more than just poke it home.