Well, that's just wrong. The thing that makes Maatta great is that he really doesn't have any weaknesses. Fine: he can't skate as well as Duncan Keith, and he doesn't hit as hard as Shea Weber. But in virtually all other respects, he's extremely well rounded, and the most well rounded of any of our young Dmen. Sure, he'll improve over time, which means he'll just get that much better. He's maybe not quite as exceptional or advanced as Jonas Brodin, but he's very close and very similar in terms of impact and being able to step in and play a pretty complete game. He's only going to get better and better, especially with his work ethic and commitment to the game, on and off the ice.
Indeed, Despres, Dumoulin and Harrington are all REALLY good players. The fact that Maatta stepped in and earned a spot over them has nothing to do with not being able to be sent to the AHL. He outplayed each of them, and that's really a huge complement and indication of just how good he is as a player, considering he outplayed some really high-end defense prospects.