With this system, and d8 as your hit die, each plus of 4 1/2 ends up averaging out to another hit die, and a 9+9 HD creature has the exact same average number as an 11 HD creature (49.5); just a tighter spread (18 to 81 vs. to 8 to 88). But, the 11HD creature is better at attacking and saving throws.
What I wonder is how much monster design takes this feature into account. There's a certain inertia when it comes to messing around with the ogre's 4+1 or the troll's 6+6, arbitrary figures seared into a generation's brains from adolescence. After a while, hit dice codes seemed to be put out there for sheer novelty value, jumping the wereshark in Monster Manual 2 with stuff like 13+39 (the arcanadaemon), 2+8 (the tri-flower frond) or 7 plus, um, 3d4 (the annis). Keep in mind that each +3 hp counts as an extra hit die when attacking, by 1st Edition AD&D rules. Math is hard!
|Or is that seven, plus three, minus twelve? Or 10-19 HD?|
OK, I find that it's hard to mentally budge those iconic numbers for the classic monsters. But maybe it's a thought when we turn to designing new ones.