Thinking more about laws & customs of surrender.
Life has value
Literal value. You can buy yours (and your stuff) with payment. Many of your foes are actually more valuable to you living than dead. They can provide ransom, be enlisted to service, and may even be socially obligated to provide information willingly (depending on the culture.) Killing them means all you get is their stuff. Sparing them may mean you get more value than their stuff is worth on the market.
"No Quarter" Means Something
If most battles are to defeat, not extermination, then the ones that do mean something. Foes that kill their defeated foes are a little scarier and deserve death in return became a special threat. If every fight is always to the death, then there is nothing special about ruthless foes.
Murder-Hobo Disadvantages Hurt
If you are generally compelled by your disadvantages to kill your foes (Bloodlust, Intolerance), or tend to limit your options in not using excessive violence (Intolerance, Bad Temper, Compulsive Behaviors of the right sorts, etc.), you will suffer an inability to take advantage of such customs.
This makes disadvantages like Bloodlust really more of a disadvantage. Right now, it's generally a quirk-level of effect. It's annoying to players who'd prefer that they question a prisoner without access to supernatural means to question the dead, but otherwise, that's about it.* But if foes reasonably expect battles to not be to the death, and gaining something of value from a defeated but living foe is on tap - even more value than if the foe is slain - then it's well worth -10 points.
Civilized Disadvantages Hurt & Help
Code of Honor, Honesty, and other do-gooder disadvantages are a disadvantage in that they compel you to follow the rules of war and surrender. But equally, they help you when the enemy has them, and will essentially force you to take the more valuable "spare them" option. If life is indeed valuable, in a monetary sense, you're going to come out ahead of the murder-hobo types.
Anything else I missed?
* Well, that and the inevitable attempt to explain that your guy puts in an extra shot to be sure of a downed foe at the end of a fight, not in the middle of battle, so it's not even tactically inconvenient.