I don't think most people pull up with more force than what's necessary to lift their leg on the upstroke. I'll argue that's still beneficial, because if my leg weighs 50 pounds, I instantly get 50 extra pounds turning the cranks compared to letting that leg just sit there.

So how does clipping in come into the picture? You don't have to worry about a little extra force to pull up lifting that foot off the pedal, or a bump or pothole knocking that foot off. Now you can focus on a smooth spin without worrying about crotch meeting top tube.