In the early game, if you get several Dancers, you can basically kill the enemy before they get near you. I haven't done this myself, but I used to read about Single Class Challenge playthroughs, and Dancers have an easy early game for that reason. You'd need several for it to build that way, though, and it's a pretty specific build that isn't as good later. You're also less likely to get Dancers that early due to high job level requirements.
Though I do agree that having one doing Slow Dance on a big map, isn't that bad late game, though there are some broken options available (e.g. Calculator)
"so is my word...It will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it." - Isaiah 55:11