Here's my problem with the OP... Most people who never heard of JAG prior to the draft will swear up and down (now) that they would have made the pick. And now that Manziel has (further) exposed himself to be a jackass, it's easy to be super smart in retrospect. But there's no way in hell.
Some of the real draftnik fans on this site who did extensive scouting of the candidates will own up to the fact that Manziel was kinda intriguing predraft...remember BB brought him in to Foxboro for a face to face, which caused a stir. And some of them actually realized then what we all figure now as obvious: BB would have been frustrated coaching Manziel, especially off the field.
But still... There's no saying that Johnny Football wouldn't have behaved differently this preseason if he had Brady as a role model, and Belichick as a boss. Gronkowski spends his summers and weekends almost exactly as recklessly as Manziel, and I'm not seeing Pats Fans vilify Gronk as incongruous with the Patriots Way. Talib was an ass on the field, too, and is still remembered fondly by the fans. Spikes had his porno thing. Welker did that media thing mocking Ryan's foot fetish. That stuff makes BB nutty. But doesn't make those players undraftable.
Myself, I was thinking McCarron or Savage all the way, and I saw Garoppolo as a small school risk that I was okay to pass on. But when BB drafted Jimmy, I was cool with that. And if BB had drafted Manziel I would have been totally okay with that, too, because he knows what he's getting into, thinks it can be managed and presumably it would have meant he believes in the upside...which is still there!
Just comes down to: BB knows football and these players better than I ever will. The fact that he pulled the trigger on Polo in the 2nd seemed like a 'meh' idea to me on draft day. But then I was skeptical when he drafted Mankins (who?... a OG in the 1st?), and so I've given up the idea of second guessing him. Garoppolo is another BB pick that got lots of criticism in the media at the time, but starting to look damn smart in retrospect.