That is why that phase is hard, having the DPS to kill the conflags and keep the tank up. They are negating every single aspect of this phase. My guess is they either have tank deaths or don't have the DPS to kill the conflags, so they chose to exploit.
Their dps does seem way slower than the group I go with, we use brd/smn/smn/blm, none of which with more than 3 coil drops too lol, so extra dps on conflags is probably the main reason. But hey they beat the enrage by 1-2s at the end there so it was enough dps. Saving healer mana is kinda pointless there since one or both healers are likely going to die during twister phase at some point anyways...