Starmie is absolutely amazing on Rain teams. Yeah, it's mediocre outside of Rain, but in Rain its an utter monster. First of all, offensive Starmie is the way to go. Hydro Pump/Thunder/Spin/Recover is the way to go because Water/Electric coverage is simply amazing.
Non-bulky Starmie does not suck at spinning because it outspeeds just about everything in OU without a boost, so it can just Spin before it dies. It can Recover any health lost due to LO against Waters, and Hydro Pump can dent Dragons. Trust me, if an offensive Rain team wants a Spinner to protect the TOrnadus that should be on every team, Starmie is better than TentaUSEMEONSTALLONLY
And so what if it isn't killing Ferrothorn. You do a good 40% with a Rain boosted LO Hydro Pump, which means that you can easily pick off a weakened one, and why are you NOT using LO on Starmie? Dragons will not "set up and rape you". Hydro Pump 2KOs most of them after SR, and you can have five other Pokemon to kill a Dragon...it's even easier in Rain. Dragonite isn't beating Ferrothorn in Rain at 35% health... You also have a 30% chance to paralyze them with Thunder.