Really?
Why should not lefthanders be allowed to enjoy the comfort and ergonomy of having the primary click button under their index finger? So that you can use their mice without having to readjust?
It always amazes me how often I see right-handers attempting to give 'good' advice to left-handers, for instance that left-handers should play guitar the right-handed way. In my opinion, people who have no experience in dealing with a backwards/mirrored world should not attempt to make themselves experts on left-handers.
I can use mice with both my hands. But not having the mouse in 'left-handed' mode when using the left hand just sucks. When I am just temporarily using someone else's computer I'll just go ahead and use my right hand (assuming it is a right-handed setup), though at the computers I use regularly I have it set up properly for left-handed use.