Dusty the Butterfree begs to differ. She tackled Giovanni's entire team once, only losing to the last one. Same deal with Venomoth, I beat the Elite Four with one. I haven't used the others though, but I'll try using them later.The Pika clones, the classic butterfly, we have like 7 of those ones.... It's insane. And, all of them are bad, they're bad and you know it. Not even Quiver Dance saves them!.
Usefullness aside, there's a lot of butterflies in the world, so having a lot of butterfly Pokemon isn't entirely wrong. There's several thousand butterfly and moth species, and each one looks different.