Don't even look at it's attack (yet). I'm here to talk about this guy's weird and annoying ability. Locking Ace Specs for both sides! Talk about a hard counter to Genesect. For decks like Darkrai, Blastoise, and Virizion / Mewtwo, the only way Genesect can win against them is by using G-Booster at least one time. Since neither of those decks rely on their Ace Spec, Spiritomb is a very good tech to consider if you think you'll encounter Genesect in any given tournament. It also trolls all decks anyway. Although it locks your Ace Spec as well, you do control when you want to play it down. (unless you start with it, in which case you can always use its attack if you want to.
Since Darkrai has room, I think it might be a good idea to run two or three Spiritombs in that deck and no Ace Spec. Since Darkrai really doesn't need an Ace Spec thanks to things like Sableye, you can use Spiritomb to really hurt your opponent. How about this trollish scenario:
You are playing Darkrai against Genesect. Genesect wants to use G-Booster, so they Red Signal and Megalo Cannon to KO the Spiritomb thinking it's a 1-of tech. Then you play down another one! (to add, you can repeat this scenario for each additional Spiritomb you play!). They just wasted their precious Red Signal, especially if that allows you to force a 7th prize.
It can also function as a Tropical Beach in the late game with its attack! I think this card is much better than people are giving it credit for, what do you guys think?