Why use RainChomp when you can use SunChomp? +2 Fire Fang in the Sun OHKO's Skarmory and Bronzong after SR. After they are gone, there really is nothing that isn't 2HKO'd by a +2 attack.
SunChomp is definitely worth testing- it handles both Rain and Sand well- and has amazing synergy with common Sun sweepers. Fire types take Ice all day, while Chomp takes Rock well. Offensively, fire types fry Steels in Chomp's path, while Chomp obliterates Dragons and Rock types. Fire types can even take a WoW, while Garchomp can return the favor by taking TWave. Sun provides the boost in power Chomp needs, while it can help take down Tyranitar and Politoed with it's powerful STAbs.
That's some pretty good synergy right there.