This really needs to be allowed as an option (maybe after setting some hidden option that says you really know what you are doing) so that you can swap the mouse buttons at the hardware level. Swapping the mouse buttons is necessary for people that use the mouse with their left hand.
Swapping the mouse buttons using the Windows control panel option doesn't work very wells as many games and even some applications (e.g., VNC, RDP) read the raw mouse buttons and don't honor that setting. I hacked my configuration files and it works flawlessly, but periodically the setting will get reset and I'll need to go in and hack them again to get it working.