I had adept 3 hoarfrost - if I had m1 I'd have upgraded thorns (if I get all 3 I dunno what I'll do). And the difference in damage is way way more than 100. Remember that your damage (whether from a nuke or a damage shield)) can be buffed while heals cannot. And if you spend as much time as my guild does farming eyes in SC you'll know how many elementals we go through and the damage increase is multiplied by 2.
The dps increase is substantial.Again you only need one druid with the m2 HoT so it has 0 affect on raid healing as long as there is more than 1 druid (and we usually have 3-4). So even if I have cold resistant mobs to face (not nearly as often as T5 though), I save power for the direct heals anyway.