It's even easier because it means Swift Swim is activated on the first turn of battle for Politoed's partner. There are also a bunch of Pokemon (like Ferrothorn) that just appreciate having Rain up as well. Speaking of Ferrothorn, it's a good counter to Trick Room on Rain teams. Swift Swim is turned against you under Trick Room, but Ferrothorn not only helps to counter it, but provides useful resist that many Rain abusers lack.
One of the more fun things to use is Absorb Bulb too. I used it on Dragonite (Hurricane is 100% accurate in Rain) for a while leading with Politoed, and many use it with likes of Kingdra, Ludicolo and others that can take Politoed's Surf easily and boost their Special Attack for free. And obviously in doubles you can abuse Abilities like Water Absorb much easier, because your teammate can basically heal you while doing massive damage to the enemy.
But really, there's so many options in Rain. You should spec some matches and see what people are doing, or just try your own theories to see how they play out.
