Say hello to the worst of Rotom's forms. Weak to Stealth Rock, a less than ideal unique attack (Hurricane would have been awesome), and a completely useless ability all add up to... a really cute sprite and a few unique sets. Rotom-S isn't ideal, but it can be pretty effective. It isn't like the other forms that have huge power moves, so it isn't as effective in that respect, but it does have a few options that make it sort of cool.
Because of Rotom-S's low power, most dedicated special walls beat it out even in NU.
*Scraf Edit: It is also a flying type with levitate. With a useless ability and the only Pokemon that fills this niche, it's not good.*