Really? Why's that so? Wouldn't you want to be more evenly distributed?
You have to force youself to work each arm evenly using dumbells. By doing so, it will help develop the smaller synergistic (helping) muscles, along with the major muscle groups.
So in general, dumbells are better than barbells. Barbells are better than machines.
Dumbells are more versatile than barbells. For example, you can change your grip position when doing dumbell curls. Palm facing your body, facing upward, facing downward or you can start with the palm facing your body and turn your palm upward during the curl.
Dumbells let you mimic the neurological and movement patterns of sports. Dumbells give you a better range of motion.
Power (the ability to drive a weight or your body explosively in one direction) is better improved by dumbells.