to me the lmb is usually the dominate button there for thats why id make it the increase button but at the same time i could see your way working fine aswell. i guess it all comes down to preference.
Yup. I think we should make him make it user-definable :D
in a level editor maybe you would use the lmb to add a tile and the rmb to subtract it
In the time it takes me to make the basic functionality mouse-driven I can have written the entire editor in a keyboard-driven way. My in-house stuff looks *very* in-house!