Smogon generally resists banning combinations (The only major incident I remember of this is Garchomp with sand veil. And Lati@s with Soul dew, but no other pokemon can take advantage of that item.)
The thing is even if sleep lasts more than 1 turn it's a free setup turn and that mildly dangerous Darkrai or Breloom can be turned into a monster with Nasty plot/swords dance. If sleep clause isn't in place your opponent won't be able to switch to another pokemon because the sleep user can put them down too.
Yeah and people tend to forget that the Sleep Clause "nonsense" actually originated from Pokemon Stadium and is persistent throughout the rest of the console Pokemon games, just none of the mainline ones.