They're merely just slowing them down. Even when I'm running a dungeon with SCH and NIN/MNK I do big pulls.
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Normally I would agree with the whole "it eats your MP" should be enough of a cost, but even then it's not much of one really. You cast flare, use conversion, oh look, depending on piety you could Fire II then Flare again.
And even if conversion is on cooldown, you simply transpose and instantly get enough MP to cast Blizzard III. Rinse and repeat.
While I agree the BLM is meant to be the AoE character for the game, at the same time there really isn't much cost to using our skills if you know how to manage your MP. So long as we can almost instantly regenerate all of our MP big skills like Flare need to have restrictions that similar skills on other classes don't since those classes can't regen MP like we do. People will still spam it along with Fire II.