The only reason why I think of Shoulders as Modifiers is Tomb Raider 2 for PSX I guess where you Modified you Movement speed from normal (running) to slow.

Know the Shoulders Are more "active" Buttons with shooting and that stuff, it'S kinda Comfortable but I like them as Modifiers and maybe 1 for dmg or something because you only have3 Modifiers in WoW (Combined they are up to 7) You could also get the right analog stick up and down to make them 2 Modifiers it's easier Came just into my mind (but it's crappy you can't do up and down at the same time -.- 2 ModCombinations less) only 120 ButtonCombinations left

I Hope I can start soon, this is going to be the coolest and most stylish WoW time I've had ^^