Not disagreeing with you, just wanted to say in case you didn't know: shift-insert has the same effect and doesn't require reaching for the mouse.
With that in mind, if they want to free up the naked middle-click button they could enroll a bucky bit. Ctrl+middleclick or Meta+middleclick.
> Having two completely separate mechanisms for copy-paste (not talking about the UI, but the underlying clipboard/selection) genuinely is confusing. It's never consistent across applications. Will I be able to middle-click paste from this web browser to that terminal, or from this emacs to that Skype window? Or do I need to select + Ctrl-Insert in this terminal to copy to that other window.
In my experience, middle click is an X thing and is the only thing generally guaranteed to work practically everywhere.
For this reason, it seems not justifiable to me.