This is a really fair question and one I asked myself a few times (as I definitely enjoy grinding DD).
What it boiled-down to for me was a couple of things.
1. I want to be able to 'lock-down' Dynamic Difficulty on AS+. Change that slider and XP is gone.
2. I definitely prefer more injuries. So, same deal here.
3. To be blunt, the run scoring environment did not seem to have quite enough variety on default AS for my taste. I love a good old fashioned 1-0 game, but when that's the norm; I take issue with that.
4. I just cannot stand the pitch speeds on default.
Once I 'take the plunge' on any one of these things, I may as well go all-out.

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