On long fights, Blizz 2 is horrendous. But while doing trash mobs which will go down quick enough and you can get MP regen before the next pack, there's no reason not to.

Example. Sastasha HM. Tank pulls first two packs (most of the time). DoT > Bane > Swiftcast Shadowflare > Blizz 2 spam > Use last two AF on Energy Drai on the last add that's alive > AF move to next pack and repeat.