It is pretty much inconceivable to me to not roll HP for monsters in old-school D&D and it's variants.
The only time it would make sense to me to not do so would be in a tournament, to make sure encounters are of equal strength for all participating parties.
I started doing this too, admittedly using a virtual die roller to get quick, lined up results. It's probably OCD of me, but it does make me think about monsters as having existence outside of the the few minutes they are "on-screen" encountering your players. For example, I had an encounter with four stirges my last game. Two had nearly max hit points—5 and 7, decided thus they were sated on something they attacked earlier—while the other two were unlucky in that they had 1 and 2 hit points—runts of the flock who didn't get in on that sweet giant rat action earlier. This made the two starving stirges a little more desperate and reckless, while the other two... not so much. The two starving stirges were quickly dispatched, an the two fatties worked out that they'd best find desert somewhere else. In another encounter I had a bunch of skeletons who wound up with 1s and 2s in hit points, so I decided they were turned and damaged by an NPC group elsewhere on the level.
TLDR rolling hit points stimulates my brain's verisimilitude ideator.
This is the best argument I have heard for rolling individual HP.
“How can I wear the harness of toil And sweat at the daily round, While in my soul forever The drums of Pictdom sound?”
Jimm.Iblis wrote:
