Actually, Swoobat didn't fare all too well against the two or three stall teams that I fought while laddering. Munchlax, Grumpig, or Mandibuzz always rear their ugly heads to stop you on those, and it's more or less impossible to muscle through them. Swoobat fared best against Balance more than anything else, or possibly bulky offense: they almost always have that one physical wall that Swoobat can set up on, and the rest of the team isn't quite bulky enough to survive the onslaught.
And again, I'd argue against needing the dual screens/Memento. It was a rare, rare time when I had both Swoobat set up and my Serperior's screens up. Swoobat simply didn't need them, and since it's not cutting off half of its health like Linoone's doing, not to mention doubling one of its defenses instantly, I still maintain that all Swoobat needs to set up is a physical wall not named Bastiodon, Mandibuzz, or Piloswine lured in. The argument for using Swoobat over Gorebyss is slightly more difficult, though, since Gorebyss is also achieving a +2 SpA and a +2 Spe... It probably lies somewhere in the fact that Swoobat can set up on choiced fighting and ground moves that Gorebyss and most other set-up sweepers not named Musharna can't. The base 114 speed doesn't exactly hurt either. :P