Well you see, I understand your point about it being a tad silly to have a Dancer saving the world from Gods, Demons, etc. However, it is an old FF job, and also the game is about party situations. You could say the same thing about Bard, Puppetmaster, Thief, and Scholar. None of them sound intimidating or inspiring, but in the game's world they are much more than just a title. Also in the real world these types of people may possess skills that aren't so straightforward in battle, but could be used to gain the upper hand.
Then again, we aren't dealing with the real world, and part of the Dancer's motif in FFXI is that it's supposed to not be taken seriously in battle. This is supposed to be part of the reason they're so good. That and they're secretly supposed to descend from martial artists who danced, at least the person who teaches players how to dance was.