I dislike the UX changes done in Gnome 3 as much as anyone, and middle-click paste is firmly wedged in my muscle memory. But this is actually a change that's justifiable, unlike the general change for change's sake.

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. Pretty much no way to know without learning it by trial and error. After almost two decades of using X I'm still surprised weekly about pasting the wrong thing due to this stuff. (And any time I paste text to an IRC terminal, I make sure to paste it to a throwaway window first to verify I've really got the right content).

And really, while I will continuously use select + middle-click to copy, it's only because of muscle memory. It's not some huge productivity boost over using the keyboard shortcuts. And once it stops working, the muscle memory can easily be retrained. (Remember how you probably complained when browsers stopped treating middle-click with an url in the selection as "open url"? I have no trouble admitting that I whined like hell about the massive loss in productivity it'd cause me. A month later I didn't particularly miss it, because I was automatically using a different workflow.)

The primary selection is a ridiculous vestige. Good riddance. I'm less enthusiastic about what they're proposing to waste the middle button on.