There's several reasons, so let's go into em all:

Transparent effect, if you have a dancer, and the unique mantle - that gives you transparent effect all the time - nothing can touch her. All the while, she's dealing constant damage, or abnormal status, or draining mp. Bing a dancer's brae up to 96, and she'll devestate on her own, then if you have more thna 1 - the power is doubled, or even tripled. I never needed more than 1 dancer... but if I was sadistic, I'd have a group full of them (I wonder if I could turn cloud into a dancer... never tried..... hmmmmmmmmmm that'd be neat ).

Then there's the dancer combinations, I know a lot of people - go Black mage, with Math Skill - I have a dancer with the Calc skill, and equip gun (Stone Gun). Anytime she can't calc, she dances - with her high faith & brave... it's just a devestating combination. I almost don't need any of my other units, even in some of the games hardest fights. It's rather... satisfying to have all your other units, stand back & wait - while the dancer slaughters the enemies with barely an effort.

Then again most of my units can do that..............

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But I love my dancer anyways. Favorite dance - Nameless Dance.