Answer:

To be honest, you can actually get a BETTER workout with dumbbells than you can with barbells. Of course, some exercises are better than others as far as muscle building goes.

Using your example, the barbell bench press is actually a better mass building exercise than dumbbell flyes yet dumbbell bench presses, I find, are better than barbell bench presses.

The reason is that the dumbbells force you to stabilize the weight far more than barbells. This increases muscle activation, thus working more muscle mass therefore improving your results. This freedom of movement is the main advantage of the dumbbell.

Barbells have the advantage of stability, however, increasing the amount of weight you can use compared to dumbbell exercises with a similar movement, e.g. you can barbell bench more than you can dumbbell bench.

A good program will take advantage of both barbells and dumbbells, mixing them up to work all aspects of the muscle.