Dumbbells force you to stabilize the weight more than barbells cause you got two free floating weights. This is good. Plus, i think its a variation of the movement, and i am a supporter of variations in lifting.
Plus, i find that my dumbbell strength and my barbell strength don't correlate exactly. For instance, strength gains on barbell might mean i am even a little weaker when i try and go back to dumbbells. I assume this means that they work the muscle in a slightly different way, and therefore both are good for overall development.
-Fatty