Since the last post lead into this sorta and I feel its relevant, I worked up a chart depicting the Attack Bonuses and Saving Throws (using the Monster's Single Saving Throw Target from Swords & Wizardry for this simple example). Onn caps Monster Attack Bonuses at +15, so using the HD progression given in the last post, this is what it looks like:
|Attack Bonus/Saves by HD - CHARACTERS|
Now, on the surface this looks pretty even, especially at lower levels where Monster attack ability outstrips the fighting-man's. Now Onn has a maximum bonus of +2 for generated ability scores, so using the most extreme example, a 1st level Fighting-man could have anywhere from a +1 (no additional bonuses) to a +4 (+1 base attack, +2 Str and +1 for using a weapon of choice) for a +1 to +4.
At 9th level a Fighting-man has the same +9, but with an additional +0 to +6 (if he was lucky enough to find up to a +3 weapon he's mastered in) for a total of +9 to +15! Clerics and Magic-users as well can get up to a total bonus to hit of +10 and +9 respectively at 9th level.
The Monsters at 9 HD have a flat +9. Now you could come up with some system for determining ability scores for them, but this would add to the bloat of the stat block. You could up the Monster HD to d10 or even d12 since they would be taking more damage more often. Or you could lower the PCs HD from d6/d8/d4 (Cleric/Fighting-man/Magic-user) to d4/d6/d3 and make them more fragile.
This is where the level system shines. Adding in the fact that PCs gain abilities more numerously than monsters, splitting their HD from their direct combat statistics allows for a more dynamic system where the extra abilities of the characters are more in balance with the power of the monsters they face.