Dumbbells allow for a greater range of motion. In general, when you lift, you only get strong in the range of motion you lift in. Like, lets say you work up to a 315 bench, but only come half way down, that doesn't mean you can also handle 315 all the way down the chest.... cause you havn't trainined that range of motion.
So, i can think one benefit of dumbbell work is that you will train ranges of the motion that you cannot get with barbell work.
I am not saying one is better than the other, i am saying they are different, and both beneficial.
-Fatty