Just want to add some reasons why such an option would be awesome.
1. Practice
You know, sometimes you just want to practice.
Though you may be able to do so by using mods such as HardRock or Easy, what would happen if you just want to practice a single parameter ? You only have two options : 1. Create custom difficulties; 2. Looking for maps with original settings that fits what you're searching for. Well, doesn't it sound very boring and/or limited ?
Wouldn't it be easier if you could just set it ON when you start practicing, then OFF when you're done ? Not only would it give you many more possibilities, but it would also grant you an amazing amount of time, that you wouldn't have to waste creating/deleting custom difficulties.
I think we can all agree that the editor doesn't stand to be used as a cheap practice mode. Making CS/HP/AR/OD toggle-able in mods would be a way better way to allow player to practice.
2. The option already exists but is way too much time-consuming
- Changing parameters can be done by using the editor
- Custom difficulties in multiplayer is done by sending a custom file to the whole room (though it doesn't allow players to chose what settings they want to play with)
As I said, everything is already merely available in-game. Still, it's time-consuming as hell, not very handy, and lacks possibilities.
^Dumb design, but that's what I have in mind. Would be toggle-able in the menu by clicking a gearwheel icon or whatever.