Are abilities that reduce combat rating etc effective at all?

[Greyloins]

Lately I've been considering the abilities in the game that reduce an enemy's combat rating, like hox' Inferno Curse and demonologist's Inferno of Amher with the Black Ashes feat. I'm sure there might be some others I haven't considered. There seems to be a consensus that combat rating is a bit of a lackluster stat, that doesn't give a noticeable DPS increase without very high amounts of it, and that weapon DPS, +%weapon dmg and crit rating are far more important. Is this also the case for enemy NPCs? It seems to me that for extremely hard-hitting mobs like the Yakhmar adds in T5 or the Custodian in Sepulcher, you can use any damage reduction you can possibly get, however if the 600ish combat rating the mentioned spells reduce makes no perceptible difference then I guess it's not worth the time to use them. Anyone have thoughts regarding this?

Also, on a similar manner, the sin Lethargy combo reduces the % damage a mob deals, though to a much lesser degree than what a bs can do. I checked the parse of some people in my guild who main sins in T4 a few months ago, though they didn't seem to have it in their rotation, even on Emperor and Imp. Is this skill even worth having on your action bar then?

[Mogsters]

crit crit crit

combat rating is poop