Most left-handed users have just fought nature and adapted to using a mouse geared for right-handers.
SwapMouseButtons is a virtual sanity savior for anyone who has to swap the mouse from one side of the keyboard to the other. That's where SwapMouseButtons can be helpful. Likewise, users can become frustrated if you have a workstation set up for sharing in a common area and they have to constantly reconfigure the mouse. How many times have you sat down at a user's desk to help with a problem and noticed that the mouse was on the 'wrong' side of the keyboard? Whether you're right- or left-handed, supporting a desktop with the mouse configured the opposite of what you're used to can be annoying.
