[symphonix]

The impression I get is that Apple want to avoid any unwanted complexity, and putting two buttons on the mouse only confuses the newbies. G.O.K. how they cope with those five-or-six button Micro$oft contraptions.
There are always stories circulating in Mac support circles of people who had the mouse pointing backwards and had learned to pilot them upside-down, simply because they didn't know any better. People are often spotted running mice up walls, or down the side of the desk, when they run out off room. In short: people are a problem. If they added another mouse button, that would stir up merry hell in their tech-support and forums.
Anyhew, my Logitech wheel-mouse optical works fine in OSX.