THM's damage overall does take MP and efficiency into account. Your damage tanks considerably when you're outside of Astral Fire so the more times you have to swap over to Umbral Ice the less overall damage you are doing. It adds up in longer fights.

Anyways, there is no question that the team is doing their best to ensure that fights are balanced so everyone can participate. They will likely not have any sort of AOE heavy battles if it would leave melee totally useless. This was a major problem in 1.0 that they have, I feel, mostly fixed.

Either way, if higher tier fights continue to require a lot of movement, BLMs will have a much harder time than melee do as they have to move around a lot and will keep getting interrupted.