Dumbbells force you to train left and right separately. If you curl a 40-pound dumbbell with your right arm, you must also do so with your left. With most machines, you can unconsciously compensate for bilateral weakness by transferring some fraction of the weight to your stronger arm.
If you are not as strong on one side, a dumbbell workout will let you know immediately. Using dumbbells properly requires concentration to maintain good form. Because you are constantly making slight adjustments in each movement, more of the smaller “stabilizer” muscles are recruited to lift the weight, resulting in a more balanced approach; Strengthening the secondary and tertiary muscles helps create a more complete physique. Dumbbells will hit those little muscles you did not even know you had. A great followup to Dumbbells is isolateral lifting using separate carriages offered on the Bodyforce X9. You can make resistance changes on the fly while taking your muscles to total failure. And nice supplement to any dumbbell workout program.
At Bodyforce, when someone asks us for the bare essentials starting point, working out on a very small budget and little space, we always recommend a quality bench paired with a good set of dumbbells.