I suspect part of the reason they made the change to monster HP is because they wanted to implement surfaces. If you have a lower HP enemy, surfaces are much more effective as they are not mitigated by AC.
Change one thing in a well tested and balanced system, then you have to change something else, then you have to change two more things... chain reaction.
The game would benefit greatly from drastically reduced surface use. Concentration spells are nerfed to oblivion when you have to wade through seas of fire everywhere.