It may have its uses, main one being its really cheap compared to +40spell dam. Also from reading on testing of Deathfrost it appears that the melee and spell ppm are completely independent. Melee acts as a straight 3-4 ppm(meaning they can refresh the debuff), while spells proc rate is aprox 50% but has a 20-30sec ICD(so aprox 2ppm). Also the debuff doesn't stack with thunderclapp or curse of tongues.
More testing is needed, if you want to follow the progress just check out elitistjerks.com
Did a lil testing of my own: Deathfrost will not proc on any pally reflective dam. Tested ret aura, BoSanc, HS, shieldspike. However, the 150 frost dam is buffed by 1hand spec talents, so the dam is actually 157 per proc(236 crit dam) if 5/5 in 1h spec(which most of us tankadins are).
After 15min of some more testing, melee proc rate seems to be between 3-4 ppm(only aprox 600hit test, lower lvl mob, no parrys/dodges, etc), so maintaning a ppm of 7 seems unrealistic as well. Procs were highly variable as well, so in practice be looking at 3-8ppm during any given minute.
Edited, Jun 23rd 2008 5:07am by mahlerite