The thing with limit breaks is that they are EXCLUSIVELY for use by Melee DPS, and are only to be used if at MAXIMUM POWER and the final boss is at <15% HP.
... Well that is a very common take on the subject, anyway.
The reasoning (aside from 'flashy finisher') is that the melee DPS limit break is a high-potency single-target attack, so they get 'priority' in this situation. (As an aside, some players can be very protective of this responsibility, and get irate if you take it from them, haha).
A less common, but very reasonable convention is for caster classes (SMN/BLM) to use their ground-target AOE limit break on very large trash pulls mid-dungeon. This is because it is fairly likely that the limit bar will fill during the rest of the dungeon and the boss fight anyway, so there's no harm in using what you have. That's why holding it can be seen as wasteful.
Of course, MCH/BRD (line attack), Healer (restorative), and Tank (defensive) limit breaks have their uses and places too.