I had a DM once that apparently ignored monster hp and just winged it every time. He did this in order to maintain control of the challenge level. If he wanted the encounter to be on the easy side, it was, if he wanted it to be extremely difficult, it was. Typically his encounters were always pretty challenging. It took us a while to catch on, but we did actually figure it out eventually and no we did really like that he was doing that. My point is, it could work in small doses if you wanted to flub an encounter that turned out too difficult or add some extra challenge to one that was turning out too easy. Just a thought.
Obviously, he never did anything silly like allow a goblin to soak up 100 damage or anything hahaha