routes are 25 yards instead of 15, linebackers have to drop even further, which opens up the underneath; they can't just sit on them. That then causes the safeties to drop even further, which opens up the run game. By the same token, as bluefrog noted, if a guy can throw that 18 yard comeback, what does that do to your routes out of the backfield to the flat?

A guy with an average arm can't stretch the defense in that way, and it's why John Beck is probably the most underrated QB we've ever had.