All the classes have different uses.

BLM shines in either large group AE's or single target low HP mobs that need to die fast. Examples, Napalms in CT, WP Speed runs, etc

SMN shines in small group AE and longer lasting single targets. Bosses and normal pull packs or 4 or less.

MNK shines in single target. Bosses.

While you can use any of the above for anything, I myself like to switch it up to optimize what I need to do. For example, Turn 1 in Coil has no special needs.. so Monk is a great choice. Turn 4, however, I go BLM to burn down adds quick.

Being a swiss army knife is not a bad thing.