The problem with him is the fact that he's NOT useless outside of the rain, it's one of few rain abussers who can actually still be annoying as hell and support it's team doing wallbreaking and stuff like a king without suffering damage at all. Yeah, it has counters and checks, but let's think about it: Hydreigon is 100% OU, and he doesn't have any counter on the entire NU to OU metagame. While Genesect had some counters, but he's obviously Uber, so, not having counters =/= being broken.
The problem, in fact, of the great to total majority of the Pokémon is trying to win a clean or good switch-in, for example Hydreigon suffers as hell by a good ammount of attacks or move if he tries to switch-in, and he's not a god-like revenge killer like Genesect, either, well, actually, the same happens with Tornadus-T, but... the difference is it's ability, Regenerator, who, useless you had Stealth Rock + A Super-Effective attack against him, he'll be able to Switch-In by an almost infinite ammount of times and wreak havoc with it's 100% Hurricane on rain, or for U-Turning, or for Wall Breaking, or just for use support moves or protect with coverage it's partners. We could say the same of the others Regenerator users, but... Tornadus-T is the only one who had legendary BST, awesome speed, inmunity to Spikes and Toxic Spikes, and doesn't had weakness to Pursuit or Dugtrio, combine it to being able to also abuse of the Rain (the most powerful weather), cover almost flawless the weaks of it's Rain Partners, and a nice moveset... and you have a monster who is able to do the job of 2-3 Pokémon in one, and better.
He'll lost at the long run, but by the moment, he did more than any other OU Pokémon.
I love Caterpie, he's a so adorable little bug! ♥ (Also, sorry for my English, I'm still learning it and I'm not a native speaker).