For me I love the sub-genre rather than just being able to list genre's multiple times. Here are some thoughts as to why
1. I would like to be able to track what genres and sub-genres I have. Just how many Hockey, Action RPG or First Person Platformers do I have?
2. If you can only list it by the general genres then a modern wrestling game could show up as: action, RPG and sports. Skyrim could be a FPS, RPG, Action, Adventure.
3. If I could sort through my collection by sub-genres then if I felt like playing a twin stick shooter I would be able to see the list of all the ones that I have and pick one that way.
Lastly I don't see what the harm is of having more choices. To me it is the difference of having Sega listed or having separate entries for Master System, Genesis, Saturn...
Just my thoughts on it though.
Also if we do have a sub-genre list I would add and adjust:
Sports: - Extreme
- Alternative
Change multi-sports to Compilation
I would change Racing to Driving and include Action(for Twisted Metal type games or Crazy Taxi)
I would move Mixed Martial Arts to Sports
Action: - Platformer(to me Contra would fall into this category)
Adventure: - Open World(for Minecraft type games)
Puzzle: What is a tradition puzzle game?
- Word