An example of a common exercise that doesn't make biomechanical sense is side bends performed while holding a dumbbell on each side. Of course, the weight of the dumbbell offsets the weight on the other side, making this exercise ineffective at sufficiently loading the lateral flexors of the torso.
Even performing side bends while holding a single dumbbell on one side of the body isn't very effective at targeting the lateral core because of the angle of force involved. There's just not much resistance. The dumbbell is very close to your body, giving you a mechanical advantage over the weight. You'd have to hold a very heavy weight, which may exceed your grip strength, to have a reasonable training effect.
So instead of using dumbbells, use a single cable that's at roughly a 45-degree angle to your body.
Cable Side Bend
This is much more effective than holding a single dumbbell (or two... which is just dumb).