Xeviat
Dungeon Mistress, she/her
The other danger is that it significantly inflates the HP totals of low-level enemies. A goblin would go from 7hp to 17hp under this system, and a kobold goes from 5hp to 16hp. A stirge would go from 2hp to 13hp.
Monsters don't need to use the same rules as players. They don't get max HP for their first HD either, and use "average HP" rather than the "average-round up" HP PCs use.
I'm likely to treat your con HP as "wounds" and the rest of your HP as "vitality points". So not giving extra HP to monsters is fine by me; they go down once you hit them good.