Bards get a healing LB because, when you need to have it used, the healers are probably getting their ass kicked by how much healing they have to put out. The bards are there to alleviate some of the weight from the healer's shoulders. Also, having a "non-healing" class have a healing limit break lets you rez healers if they both happen to die to a certain mechanics, without having to wipe necessairly.

Also swords or comets doesn't make sense for bards.

Bards = lift spirits of the party, when they introduce another job for archers, along the lines of sniper, then you'll get your single tgt LB