i would argue is there a point to lifting 'light' if you are natural.
take a look at my first post on the best rep range thread.
truth be told, any weight over 65% of your 1rm will increase protein synthesis, and tear muscle down somewhat - allowing growth, however studies show the greater the weight, the greater the strength gains, and the greater the protein synthesis response.
so its more of a question of are lighter sets optimum ?
the theory of the pump or burn being necesary for muscle growth is still hotly debated, however there is no reason not to try it by throwing in a pump/burn set or 2 after your heavier sets to fill the muscle with blood, and get the full metabolic benefits ( if any).