...cause dancer isn't a heavy DD job. I play as dancer quite a bit myself, what dancer has going for it that other jobs do not, is its sustainability, and unique ability to heal without consuming mp, or having spells be interrupted. Sure it is a DD job, just nowhere near the extent of say, Samurai for example. It's an excellent job for soloing, and even a good all around job in most party situations.
Even with a White Mage in most situations, having a dancer isn't a bad idea for when the WHM runs out of echo drops, healing waltz can be a life safer, or in a pinch when you might be about ready to weapon skill and need to toss out a divine waltz 2 to cure/wake up the rest of the party.
It might not do the most damage, but it still plays a unique and important role in many situations.