Remember that Summoners and healers use BLM cross class, which includes those abilities. Those Blizzard 2 buffs would take it from 50 potency to 170 or 290 total for an AoE. That is quite a bit in general, and may not feel like it to a BLM (I don't play BLM so I wouldn't know), but it would be a lot for other classes. For a frame of reference, SMN and SCH have their own AoE around caster with DoT ability, Miasma II, which has a whopping total potency of 70.