Job Individuality/Pride/Dedication

[Soukyuu]

most of the time we were looking for either a whm or a blm. we can't really bring a brd in to replace a whm. and a brd can't replace a blm as a ranged DD. the gap in DPS is just too great.

You don't want to bring a DRG/MNK in place of a BLM to great buffalo. because they will get slaughtered by the AoE. You can't bring a WAR tank to a Garuda burn because the war will not be able to generate enough enmity to keep the BLM alive.

So summing this part up, it's "because it doesn't work with this strategy". Why not try a different one? Why does it have to be a BLM burn? Because it's the fastest? Safest? Always repeating the same strategy is boring.