See, when Rapebrerger is injured, he's still an effective passer at 70% mobility. Same with mannings, brady, etc.
These young kids like Tebow, RGACL3, etc... Once they lose 5% of that mobility, defenses can just fade back and dare them to throw, and pick them off all night. RG3 has a brain, whereas Tebow is probably 90 IQ, so he may watch film and dominate. But this whole "mobile QB" thing only lasts when the QB is 100%, and that doesn't last 3 years.