A lot of the spells in FFXII are interesting and have unique uses, especially Arcane magic. But Decoy is awful.

A lot of Blue Magic is lame too. Goblin Punch or Magic Hammer, for example. Roulette, however, might be the only spell that is worse than useless. There is almost always more party members than enemies in any given battle, giving you about a 20-40% chance of attaining a neutral or better effect and a lot of the times the enemies that you would actually use it on, like bosses, are almost always immune to it. It's the only spell that, by circumstance, is more likely to harm your characters than help them.