The hard thing about battling a Slowking in the old days was not their own power – for they were never anything impressive – but the fact that pokemon must battle them without being able to count on their trainer. The psychic waves emitted by a Slowking disrupt the communication between pokemon and trainer, filling the pokemon's head with such confusion that they only hear half their trainer's commands. In some battles, this is no issue at all. With a few exceptions, pokemon are not stupid, and they know when to use super-effective or simply powerful techniques; it is with good reason that those who command pokemon are called trainers and not tacticians. They are not an invincible pokemon.
What Slowking are, and why the League banned them unless they wear a frequency-altering item, is an anti-strategy which can counter every strategy. Thunder Waves paralyze pokemon, but Slowking paralyze the trainers, and it is after all the trainers who make the rules. Yet it is hard not to side with the trainers. Pokemon battles of the Slowking era were as dull as the Slowpoke from which they evolve, a simple trading of attacks with none of the dramatic flair, clever techniques, or even tactical retreats which have made these battles beloved around the world. Double and triple battles were even worse, with multiple Slowking being used for a truly terrifying disruption; indeed, many trainers used a Slowking simply to counter other Slowking! Something had to change.
One must weep for the Slowking who were abandoned by their trainers after the ban, and only a few brave souls still use them with frequency limiters today. But one must also remember that banning Slowking not only saved the League; it saved the great game of pokemon battles.