Simple, yes! That's exactly it. I wanted keep in line with OSR and Shadowdark design, which is simple and straightforward. No cluttery byzantine rules, let's get right to the point.
Arbitrary? No, at least that's not the intent. I had a couple of design ideas in mind with these rules.
- Add flavor to Shadowdark by rewarding monster kills but not to distract too much from the core idea of Loot XP. So you get some, but not more XP than the Loot will get you.
- Don't encourage players to become Murder Hobos, so you don't get XP for killing monsters that are weaker than your PC.
- Avoid book keeping. When you reward XP at the end of a 5e game, it's like going over the restaurant bill to add everything up right -- "OK, who killed the Kobolds? And was it really 11? OK, carry the one from Column A to Column B...divide by Party size,...do I want to multiply by CR for extra XP?" Give out simple points with simple math.
- Add in some old fashioned party contention with a Keep What Ya Kill rule. That's the only arbitrary rule you can easily do away with, but it is designed to add party tension. In the ODD and early RPG days, even our long-time group of gamer friends could compete for XP and treasure. I forget which game, but there was an Old School RPG that rewarded XP only for personal kills and personal treasure. Boy did that make for some interesting games!
Thanks again for reading it over, I hope this helps explain why the rules are the way they are.