Dropping weight can be helpful especially if you are lifting heavy and can’t get the weight down without dropping it. There is a big difference between a necessary drop and an unnecessary drop.
Dumbbells and kettlebells should never be dropped because dumbbells bounce and that can easily hurt someone. Also kettlebells will easily damage when you drop them from overhead to the ground.
Barbells on the other hand can and are meant to be dropped but only when necessary. The only reason you should be dropping weights is for safety and not for fun, convenience, or out of laziness.
You should always drop weight instead of risking an injury. Part of getting stronger includes being able to safely lower the weights to the ground instead of dropping them. If you cannot control your weights to go down then you need a lighter load to help prevent injury.
Dropping the barbell should only occur because of three reasons:
Dropping the bar from overhead should only occur because you failed a lift of have lost balance or control of the bar. This should not happen for regular reps during a workout.
Dropping from shoulder height should only occur when fatigue is setting in and it has become difficult to maintain a tight midsection.
Dropping from waist height is generally okay once your core and body has fatigued.
Always select weights that you can keep under control. Never drop an empty barbell or a barbell with light weights on it (2.5, 5, 10, or 15 lbs). Never drop a dumbbell or kettlebell. Always respect the equipment and the safety of the people around you!