I believe Steve Jobs hates buttons. The original mouse at Xerox Parc had 3 buttons, but Steve decided he wanted 1.
The idea, expressed by Alan Kay, was to keep your fingers on the same device (the mouse) as long as possible. I believe they even experimented with 7 keys.
The fact is that a one-button mouse has a shallow learning curve, but little productivity gain. A multi-button mouse has a somewhat steeper learning curve (15 minutes, instead of 5 minutes), but a much larger productivity gain. The 10 minutes you loose, you gain manyfold if you use the multi-button mouse.
Many people think that Apple did a poor job with both the hockey puck mouse (the worst design ever for a mouse) and perhaps just as bad a job with the Mighty Mouse, which, unlike his name, isn't all that great.
If you're on a Mac and you're still using the standard mouse Apple gave you, go buy a Logitech or a Microsoft Intellipoint mouse. You'll thank me for it.