Defense is a team thing. I'm firmly on the side of supporting Haluska's defense and I'll trust that he's doing what GMAC wants. I'll address a couple things here.
If GMAC thinks Sean is the best besides Clark, then he's probably guarding the best offensive player. Its quite possible that he doesn't want to have Sean play much help defense as it will open up a guy that can hurt us more.
As for denying the passing lanes, its a great thing as long as you have the skills to do it and the rest of your team is good on help defense. If your help defense is weak and you are guarding a quicker player, the other team will burn you on the back door too much. Maybe Brackins can help on the backdoor, but our other guards are slow on the rotation to cover Brackins' man. Maybe Sean has to protect against having new players learning team defense. If the other players haven't figured things out yet, then everyone has to compensate and maybe thats why Haluska isn't doing the things all our message board experts are demanding.
Maybe Sean isn't the best defender GMac has seen, but maybe, just maybe he does what GMac wants better than the other players. Garrett has all the ability to be a better defender that Sean, but maybe he just hasn't grasped the concepts that GMac is trying to teach him yet.
All I know is this, when Sean is in there, good things seem to happen on the defensive end. He gets his hands on passes, causes problems for the dribbler and generally plays good defense.