Dumbbells and isolation exercises have their place. Dumbells allow for greater range of motion and therefore more flexibility and range of contraction of the muscle than a bar does, I can really feel the difference between bench press and flat dumbbell press. Isolation exercises are meant to also build mass, but primarily to sculpt and shape the muscle into symmetrical and aesthetic shapes. If you do only mass building exercises, you will end up with a clump of muscle for what should be a bicep. Barbells and heavy weights are definetly used for mass and strength gains as mentioned.
By the way, great info Ross.