I think this conversation is way overblown. Yes, I think we'd all like to see him have a bit more weight but it's not like it's a good idea for him to just develop a gut, that's not going to help him one bit. It's not the biggest factor by any means in whether or not he can start. I was one of the people saying that no way he should start as a true freshman by the way, but he's had time to develop mentally and physically and he should be more prepared.
The truth is Coker was the only stout quarterback Nick Saban has had start at Alabama. And yes, it's nice if a guy can have a Cam Newton/Tim Tebow type build, you feel like they can take some hits. I don't think that's really necessary, people saw pictures of AJ's chest tattoo right? Guy wasn't buff, guy wasn't stout, but he won championships right? He stayed pretty healthy to. Out of the current quarterbacks, the only one that I think would fit this idea of having a thicker guy that can take licks is may be David Cornwell. I'm not sure if he's put on any weight since he got on campus but he seems to arrive with the build people have in mind.
Here's the thing. Brodie had an injury history before he got to Alabama. Just being skinny doesn't make him tear his ACL in high school! Sure, you can argue he'd have taken other hits better but keep in mind he was injury prone, period. What about Coker though? He suffered a season ending injury in college. Cornwell suffered a season ending injury in high school. Does Barnett has a history of injuries? If not I think this is more an imagined problem than an actual one. AJ was a skinny guy that didn't have a history of injuries and he had a great career at Alabama.
The real question, the real issue if can Barnett read the defenses and can he make the throws. If he's the man he should play, and if that's the case and everyone else does their job he can win a championship, at his current weight.