It's been part of the evolution of the pro game. Pass rushers have gotten so good that it's no longer realistic to expect an offensive line to consistently maintain a clean pocket for four seconds and change. The QB's own running skills, these days, is part of what keeps pass rushers off-balance -- charge too hard, and you're giving the mobile QB an easy outlet around the formation to a big rushing gain. Sometimes a team has to spy a good rushing QB -- that's one less guy available in either the pass rush or pass coverage. Needless to say, it's changed the game.