I've wondered this myself. For me, the weights used are too light to help my lower body, but the upper body moves are too fast to go much heavier. Plus, I often feel unbalanced when doing compound exercises, like my form is really poor for these moves. I was thinking of substituting in heavier lower body work and ignoring the upper body piece (which has nothing at all to do with your question, now does it? Sorry about that!).
Shelbygirl