We are seeing more and more running QBs in the NFL. And there is no question that it puts incredible pressure on a D by adding another player to cover. Most Ds will need to change their game plan (spy, etc.) to handle the extra dimension and in doing so will weaken other parts of their D.
However, I believe it has an inherent flaw. The more a QB runs the more he will get hit. And I think that the same sort of minor injury to a wrist or arm or hand or shoulder that would not really impact a running back could be disaster when throwing the ball.
Would love to hear other peoples thoughts.