I like to think of it like rock-paper-scissors. Bard auto attack beats caster, caster nukes tank/melee, tank/melee punches Bard in the face.

Yeah, there's more complicated steps in there like crowd control and pets and stuff, but in the end, someone will come up with a theorycrafted perfect party comp before PvE players figure out they need to work as a team. Got 2 healers? Take 2 Bards with you to interrupt casting.