Keyboard shortcuts are not standardised and we get on just fine. Yes, there are some set standards (Ctrl+C, Ctrl+V, Alt+F4, etc.) but then there are also shortcuts that are specific to one application. For example, between The GIMP, OpenOffice and Firefox there are loads of keyboard shortcuts using the same key combinations but with completely different meanings.
Keyboard shortcuts are considered “advanced features” by most users as they are something you have to know and learn. I see no reason why the middle mouse button cannot be the same; if you know what it does, use it.
“If the first and second mouse buttons have standard, well-defined meanings today-- why can’t the third button, too?”
Well, what function would you map to it? Selecting and context menus are used in 99.9% of applications, but what other features are universally implemented? X’s choice of paste was a good one - it is a repetitive task, and is a function used in 99.9% of applications. The autoscroll comes a close second (there are lots of applications that involve some form of scrolling), but controlling tabs makes little sense. In a non-tabbed application, what would it actually do?
“I hope over the next few years Microsoft and Apple can decide on a set of standard middle mouse button behaviors.”
Why just Microsoft and Apple? The open source community deserve a say too