That's actually a really great example and I didn't consider it at all. Henne clearly lacked the talent, and obviously sitting or starting wouldn't have made a difference. Nice point. -EK
I don't know that I would say that Henne lacked the physical talent to be a good QB. It seemed to me that he had the tools, and just couldn't make the correct decisions. Therefore, one could argue that if he had been thrown into the fire, he might have been forced to think for himself on the fly.
Obviously, you can never know for certain; that's the nature of this hindsight evaluation. But there's a difference between players like Pat White (who are just not physically up to the position) and Chad Henne. And it's impossible to know whether starting earlier (or sitting another year, for that matter) would have made a difference.