I guess it depends on the hardware used – I mostly use one of these external Thinkpad-like keyboards with three dedicated buttons and a track point and never accidentally middle-paste.
But I guess if middle-click never does anything useful for you (Windows, Mac), you might prioritise it less in your hardware design :|
It does do useful things on Windows. There are the browser behaviors I mentioned above. Most image editors use middle click to pan, and I think 3D modeling programs use it to rotate and pan as well. It's only in dedicated text boxes that middle click does nothing useful, but we're all so used to Ctrl-C and Ctrl-V that we don't have any desire to use that button for pasting.
Certainly, but those are all related to holding the middle mouse button and moving the mouse, not clicking per se. Similar behaviour is available on X with something like:
But I guess if middle-click never does anything useful for you (Windows, Mac), you might prioritise it less in your hardware design :|