Just because a guy can gun it doesn't mean he has a good arm.
He is not accurate, it takes accuracy to have a good arm.
He also has trouble reading defenses. It takes an ability to read the defense to turn a good arm into a great arm. That's why Max has a huge advantage at QB over Locker.
Don't get me wrong, I really like Locker. He is exciting and fun to watch. But unless he learns to think at QB instead of just running, durability will be a big issue.
Locker has the athleticism to be a QB like Vince Young or Steve McNair. But I just doubt he'll ever reach that level.