The reason why Pain Elementals are considered to be "christmas came early" for MVm runners is that each kill brought back 3% or whatever to your total health, regardless of what you killed. Well, USED to...as of 0.9.9.7 that's no longer the case. Now you get a fraction of health based on the max health of whatever it was that you killed. Lost Souls not being very meaty monsters, well, they have very minor healing returns now that MVm's changed.
Also: "Why are they trying to melee me?! That's not supposed to happen!"
You can force any enemy and every enemy in DoomRL to melee you if you get yourself in their melee range. Pain/Agony Elementals have a tendency to shy away from you while spewing their Lost Souls/Pain Elemental offspring to deter you but you can force them to use exclusively their melee attack if you shove your face against theirs. Needless to speak for the melee-only enemies like Demons and Lost Souls, like.
If you're playing a Melee build then you should expect enemies to melee you back. :)
I don't know what else to say for "Pain Elemental farming" since I've never personally done it...maybe tried it a few times in 0.9.9.6 when the MVm life leech mechanic hadn't been changed.