There are similar situations like Ifrit Hm being very hard to beat if you dont have a mage limit. This doesnt mean that every DD should have an additional AOE LB, rather maybe the DF just shouldnt build parties without those classes to begin with.

The DF actually prioritizes to give one dps slot to a melee and the other to a range dps, i feel for some dungeons it might help if this was mandatory, i dont think the queue times would be that much longer.