They're meant for destroying heavy bombers, but they take a lot of practice to use correctly and effectively. Their purpose is so specialized that very few people take the time to master them. How many times have you upped a fighter for the sole purpose of buff hunting? Buffs are pretty sparse now, so the 21 cm rockets often go unused.
Still, I've heard that when used to maximum effectiveness, a pair of those rockets will destroy an entire formation at once.
