(Assuming dots are already spread and being upkept)
As SCH, Blizzard II is DPS gain on 4+ mobs vs the other gcd options Ruin/Ruin ii/Broil
As SMN, Blizzard II is DPS gain on 5+ mobs vs the other gcd options Ruin/Ruin ii/Ruin iii and more cost effective than an out of dreadwyrm Ruin iii
As every other magic user, Blizzard II is a DPS loss.