Punching with weights doesn't develop power. Think about it -- gravity is always pulling the dumbbells straight down, and that's not conducive to punching power.
I took boxing/kickboxing lessons for awhile (hoping to get back to it), and the coach had us do punching exercises with 1 lb. or 2 lbs. dumbbells, never heavier.
My undertanding was that the purpose of this was to improve the endurance strength in our shoulders (particularly the front deltoid muscles) so that we'd keep our hands up during training and sparring with gloves.
At our gym, everybody had to wear 16 oz. gloves to spar. Well, 16 oz. = 1 lb., so in effect, whenever you stepped into the ring you were punching with 1 lb. weights attached to your hands. The dumbbells just replicated this. For people who wanted a little more challenge, the 2 lbs. dumbbells were available.
I can't see any sane reason for punching with dumbbells that weigh more than 3 lbs.