A lot is based on a few high profile cases that seemingly validate whichever theory you back (good or bad). Not sure any serious study into the topic exists. I do think there are a few main theories that try to at least ascend simply anecdote; 1. Their confidence ruined. "This guy is gonna be so overwhelmed in the moment. You need to put him in a place to succeed or his confidence will be crushed" I personally find this explanation crap. If he can't handle adversity, he isn't a franchise QB 2. You'll get him killed This is basically the David Carr defense and also used anytime it isn't an all pro line established. I generally find this to be bunk too, though there could be an extreme case where it was applicable, I suppose. 3. He needs to learn an NFL offense/reads and/or mechanics. This is similar to 1 in the sense that you're worried the guy will be overwhelmed by the moment, but rather than his mind, you're worried about his skillset. Learning on the fly, a guy may develop bad habits, relying on whatever his particular skill set is, whether than his cannon for an arm or his legs/escapability, etc. rather than developing solid NFL QB skills that will consistently allow him to pick apart NFL defenses. I find this the most convincing argument in general terms. However, whereas it's probably a concern for say, Watson right now who needs to develop a lot of skills, I am not concerned with this line of thinking on Trubisky from what I've seen- his skill set is already pretty advanced, I dont see the big potential for bad habits to form as he learns on the fly. It also probably helps that he doesn't have one overwhelming physical talent he is going to lean on too much. Plus, at the end of the day, the competition at QB is just too lackluster. After all, with most of the success stories on making a QB wait, they sat behind legitimately good talent.