There are few bad exercises, there are many which are better or worse for particular goals.
For beginners, the main usefulness of curls and kickbacks is that it may make them happy to do them, and a person happy in their workouts will stick to them, thus keeping up with the actually useful exercises like squat, deadlift and overhead press.
Curls and kickbacks for beginners are like when you tell your kid they can have cake if they eat their vegies. It does actually add some nutrition to their diet, but the main benefit is that it gets them to eat their vegies.