Agreed ... sorta ... The mouse has always be the primary tool for invoking functions from the menubar. But the Mac OS didn't recognize the concept of a "pop up menu" until maybe System 7 (maybe earlier, but definitely not the original OS). Without that, it's hard to imagine Apple engineers figuring out what to do with an extra mouse button.
Didn't the Amiga have a multibutton mouse? I think Digital (or a VAX OEM) was selling workstations with possibly five button mice. (Wasn't there an option for a two-button mouse for the IIc? I remember some talk of internal strife over the issuue.)
As revolutionary as many of the original Mac design elements were, some were bonehead mistakes. The first keyboards lacked cursor keys, because you were supposed to use the mouse for EVERYTHING. They touted the ergonomic design of the original toaster boxes, but forgot that as soon as hard drive makers realized how easy it was to put a drive UNDER the box.
The one-button mouse (and the lack of a fan) are still sparking debate. I do miss the original mouse; it fit nicely in your hand. And I wouldn't be surprised if Microsoft came out with the two-button mouse just to have something to argue with Apple over.
mt