Actually...drain life isn't more mana intensive than shadowbolt. yes it costs more mana per cast, but it also lasts longer. Shadowbolt is 10% base mana every 2.5 seconds, drain life is 12% mana every 3 seconds. It kinda evens out.
and compared to zero points in mana feed, 2% mana returned for every crit does reduce life tapping by alot.
I was talking about heroic magmaw, not normal. in heroic there are bone constructs created which are generally kited around. And yes, generally the kiter has their own slows, but they are generally not 30 seconds in length. i.e. I've seen vids of using frost mages to kite nef's adds in P1. Frost mage has great slows, but none of them are very long lasting, and it's possible that some of the adds doesn't get slowed. If I have CoEX on all of them, then even if they aren't slowed by 50+% by the frost mage, they would be at least slowed 30% by me.
And yeah, demo is better aoe...but I hate demo....it is stupid. and dumb.