There are bards in the game now, but not currently any that we see fight. This doesn't necessarily mean they can't. It also doesn't mean they must fit into any specific pre-existing archetype. It may be true that bards in other games are buffbots or rogue clones, but WoW is not "other games." If they choose to make a bard class, due to the lack of in-game precedent they have latitude to make it just about anything. Bards are a blank page waiting to be written on.
The suggestion that magic music doesn't fit in the setting, however, must be made in ignorance of all of the music and sound-based spells used by both players and mobs in the game - and I'm not referring to the Tauren Chieftans here. Priests use various hymns which have effect specifically because they inspire the listener. Warriors use shouts. Death Knights blow a horn to apply their buff. Those leather-worked war drums back in TBC which buffed the raid. None of these are out of place. Then you've got the sound-based enemies, such as the boss in Halls of Origination whose single most destructive attack was a song, or the various sound-based spells in Heart of Fear, also extremely dangerous. There's even an elemental lord of sound, capable of destroying worlds with a mere whisper.
The word "epic" is derived from a Greek word meaning speech, word or song. Song magic in folklore is ancient, powerful stuff. Tolkein's Middle Earth was literally sung into existence. It doesn't deserve the stigma it seems to have gotten.