A little bonus with the buttons is... because you can have them have different functionality in different apps you can kinda sneak in a "3rd button"
The default setup is that the 2nd button launches the preferences panel for the navigator which is very handy for making adjustments while you're using it. As handy as that is, I wanted different functions in Sketchup so I had to "sacrifice" that functionality and switched the number 2 button to go to X-Ray view instead.
BUT if I click on the desktop and have the Finder currently active, pressing the 2nd button still brings up the preferences panel. So it's almost like having a 3rd button. π