I’m fairly certain the combat system works on a percentile basis.
Imagine you’ve got a set of d100’s in hand. If your Melee skill is 50, you need to roll the d100 equal to or under 50 to hit a target. It’s a 50% chance, basically. 65 weaponskill is 65% and so on.
Then, I think Defense is subtracted from that. So if a battle brother has 70% weapon skill, but the enemy has 5% defense+15% from a shield (20%), then you’ve got a 50% chance to hit.
Certain things mod this, such as range for missile weapons, or Overwhelm for melee, and certain skills/weapons as well (spears, for example).
If this is true, leveling melee defense over 90 would still help in certain situations, but you’re probably looking at some serious diminishing returns by that point.
I’ve found that Initiative is basically a shit-stat. After some experimentation, I’ve never put a single point into it. The same for Ranged Defense.