I sometimes get in to parties that seem to think that pulling multiple groups of targets in dungeons like Brayflox HM should just be a standard. There are actually some parties that it really doesn't make sense for.
Example: when there are 2 melee DPS in the party.
In this example, it certainly won't speed up the rate of killing, and might even make it slower. If I am a WHM, I can contribute with Holy, but not if the tank's HP is dropping like F-bombs at a frat party. More importantly, Monks, Dragoons, and now Ninjas aren't going to kill any faster with more targets. With the tank investing enmity in more targets, they may not even be able to reach top DPS on a single target without topping enmity.
As a rule of thumb, if there is a BLM in the party and everyone is willing, I pull multiple targets. Two BLMs and it is a no-brainer. Bards are also a nice choice for pulling multiple groups, but with diminishing returns.