I don't mind spotting a guy whose lifting more than I can handle if he knows what he is doing. My old training partner is a nationally ranked powerlifter with, at the time, a 525 bench. If he failed, the most you had to pull was about 10-15lbs, the rest he could get. People should have an idea of what they can lift, and not try to go too far above that, anytime you have to pull more than 10-20lbs of the lift for them, then they had no business lifting that much.