I'd only trade him for an upgrade at PF. I'm not sure of ATL's plans with Collins, but would we have interest in matching whatever FA offer he gets if we traded for him? Of course Theis isn't enough, and isn't even required since we have the Kanter TPE, but I am in no rush to trade Theis. I also don't think he will break the bank next season...especially if he keeps up at his current pace.
His fit isn't perfect, but injuries happen. I don't think it's necessarily a crime to have 3 quality bigs to throw at opponents - especially when each of them have different strengths. I agree, though, he's not playing so hot as a strictly perimeter player.