If you've ever watched a total newbie (Grandmothers and five-year-olds, for example) try to use a two-button mouse, you'd understand how confusing it can be for them. It takes time to build that "muscle memory" that tells you which button does what. Newbies will click with two fingers, or put their left finger on the right button without looking, or put one finger on both buttons. It's a mess, and very frustrating even to watch.
On the other side of things, let me relate a story...(cue the ripple effect and harp music)...
A while ago, I was editing a large amount of text on a PC. Lots of cutting and pasting, so of course I was using the right mouse button a great deal.
When I got back to my Mac, I wondered about the experience. Why didn't I feel less productive with my one-button mouse? Maybe I'd be better off with a two-button mouse. I decided to pay closer attention to how I'd been working on my Mac.
Eventually, I had another intensive text-editing job to do, this time on my Mac. What I noticed was, instead of the select/rightclick/cut/click/rightclick/paste I did on the PC, I would just select/drag/drop the text on my Mac. I'd never really noticed, because both behaviors were so instinctive on the respective platforms.
My Mac still has a one-button mouse. In fact, i just got one of the Apple Bluetooth wireless mice. The price just came down. :mac: