According to the Bulbapedia page for Stench in Gen III - IV it has no in battle effect. That is all well and good, but it goes on to say that
From Pokemon Emerald onwards, if a Pokemon with Stench is leading the party, then Stench reduces the chances of encountering wild Pokemon by 50%.
So what did it do before Emerald? Did it do nothing at all, or did it have some effect that Bulbapedia doesn't list? If it did nothing at all, then why did GameFreak bother to add this ability at all? (and yes, I know: GameFreak Logic :P)
Here is the Bulbapedia page for Stench