I couldn't use a one-button mouse, especially when playing WoW, the right click button is used so much.
I would understand Apple making a one-button mouse if there were no right-click menus in Mac OS X but there are tons of them (obviously!) Seems pointless to have right-click context menus and make people Cmd or Option click it (or whatever the key combo is). And what makes it even crazier is that if you plug a 2-button mouse in, the right click works as it should on these menus!
I admit, I used that key combo with my PowerBook's trackpad (and still do) but now I have a two-button mouse it just feels so wrong having to hold a key and click the mouse to get a context sensitive menu.
Until Apple make a two-button mouse (preferably with a scroll wheel) they're just going to keep losing business to Logitech and Microsoft who make them!
Just my 2p.