I'm on a high-pop server, and they are camped 24/7 by a minimum of six players at any given time of the day or night.
One day I logged on my char that I had parked there, and there it was literally standing in front of me. Must have just spawned. I snagged it, though it was poor. Then I used a stone on it.
A few months later I made a DK on a dead realm, and I saw from one to ten Minfernals a day. I saw several rares, just not the H/H I was looking for. It took about three or four days to get my H/H Minfernal; And I set free the one I wasted a battle-stone on. I hope to get a H/H Blackfuse Bombling one day to complete my explode team.