I am guessing you use mouseover macros and hot keys?

The "obsession" essentially comes down to there being five fingers on either hand. While it's perfectly possible to use keybinds, and then shift-keybinds and then ctrl-keybinds, etc you're using all of this bandwidth on your left hand, while your right hand becomes a dulled single finger. I do things like put my dispell buttons, pain suppression, and binding heal on my mouse, to keep both hands in the mix.

If I were really serious about optimizing, I would put all of my instant casts on the mouse and all of my other spells on the keyboard, because I don't like trying to run and move and cast at the same time.

I hope this helps you in understanding at least some of the reasons that multi-button mice exist. They are a convenience, and can be enjoyable to use in play. It's perfectly possible to use a trackpad or an eraser mouse, or (what I used to use) a trackball.

I was using Healbot and several combinations of shift, control, and alt. I've since changed to Clique and default frames, but with a 2 button mouse (Magic Mouse).

I fully understand why people want buttons, it's just a different method of input and that's just fine. I just like to brag about healing well with a one-button mouse in front of people who insist you absolutely must have a 30-button mouse to heal at all :)