The problem isn't that you can't attach multi-button mice, which you could do even before USB came along, but, rather, that the built-in mouse has only one button. What's the point of having this sleek laptop if you need to carry an external mouse along with you. Cheeseball.

Fortunately, there are applications that can map keys to button presses, so you don't have to do that. The Enter and Option keys to the right of the spacebar are excellent candidates for this.