how many have forgotten that "the mouse" is a nick name for a "pointing device"? such a pointing device has two functions: it has to be able to "move" the pointer and "click" what has been pointed, and that's it. nothing more is essential. apple "one-button mouse" is a perfect "pointing device" by definition.
macintosh interface has benefited from this in such a way that it doesn't care about the type of the pointing device - it can very well be a touch screen instead of a "mouse", and the os (and many of the software written for mac) doesn't have to do anything differently. there are gui elements that are pointed and cliced, and that's it.
compared to windows interface where it is absolutely necessary to have such a pointing device that can perform two different kinds of "clicks". now if you replace the "mouse" with a touch screen, you're in trouble. how on earth will you tell the touch screen that this is a click performed by index finger and this is a click performed by a middle finger? compare the ease of use of any interface by imagining you're using the system via touch screen, and you have pretty good usability test of the interface. if it works intuitively and fast with the pointing device only being able to "move" and "click", then it's a good interface. if any of the functions cannot be used with these two actions, there is a design flaw in the interface.
apple has done a pretty good job regarding the usability of its interfaces, and the "one-button mouse" as a pointing device is one of the reasons for that. of course there are shortcuts, but they are considered "extra", and there's nothing wrong if someone wants to choose a different kind of a pointing device. it's just that apple needs to stick with it, because it is so fundamental to them. period. it is perfectly ok that apple sells only the mice it chooses to.
and, by the way, i consider myself as a "power user", but still prefer the "one-button mouse" as a pointing device. i also have a "two-button mouse with a scroll wheel" kind of a pointing device made by logitech, but i find the apple wireless much better than that. and i mean better, always, even when working with protools or photoshop.