To the best of my knowledge, Sleep Clause exists because Sleep is a generally overpowered status condition. The opposing Pokémon can do literally nothing for at least two turns (if you're lucky), and if you switch out to deal with the Sleep-inducing threat and it puts that Pokémon to sleep as well, you're looking at a generally broken strategy. Sleep allows for instant free switches, stat buffs until the cows come home, etc, etc. If Sleep Clause isn't instituted then I might drop out. Why not run Spore on everything if it's allowed? That's sure to be sportsmanlike.
EDIT: I'm aware only four evolutionary lines can learn Spore (five if you include Smeargle) but come on man