I work at are fitness center at school and i see many people workout. Everyone bounces at the bottum of the bench. Im guilty of that sometimes as well. How you all feel about that?
I just look at it this way: broken ribs from CPR are very common, and that's just a person pressing on the sternum, not 200+ pounds of weight dropping a foot..
It's not only bad form, but dangerous as well, as Corny's post attests. Like you I see it all the time and just look down and shake my head. It's a terrible habit to get into.
Exploding on the upward motion is ok. However, you want to make sure that that motion is controlled as well. In other words, if you're using any kind of momentum at all, you're not getting the full benefit of the lift.
It's like Bignate73 says in one of his precious few moments of genius , "If you can't stop your motion at any point during a lift, you're not lifting the weight, you're throwing it." I probably missed the exact wording, but you get the point. Control is key.