See I don't get this at all. Why wouldn't you have your best dps on firebrand burning him down where they can deal with the third add and there are no dps mechanics like DD. Finally at <20% stage you have more people on stormcaller to take DD which is more important for NiM EC than HM.
The other thing is to find the best method to deal with Incinerate whether by Stormcaller tank taunting at his guns to sky move, the taunt ***** taunting at the same time or calling a tank a swap. By his guns in the sky I mean he does the red circle barrage followed by 3 rocket attacks on the tank then points his guns to sky and launches something at the healers followed immediately by the incinerate cast.
We usually run with 2 slingers, and one of them is on SC, so when we swap a DPS over for the final 20% he just shoots through to FB so he's still on SC to eat DD.
The reason we do it that way is because of our understanding (or really misunderstanding) of the fight mechanics early on where it was believed that SC had to hit the even %s first to keep the lightning from landing on the SC DPS if you got a second DD. And now we just keep doing it that way because of habit and it works for us.