I'm gonna be honest, I don't like middle-mouse paste because I use the middle mouse for too much other stuff, and because it goes alongside select to copy, which tends to overwrite the paste buffer at the least convenient time.

But turning it off (by default) because it's not how Windows does it is fucking stupid. It's exactly the kind of stupid decision GNOME has been making for decades, and why I no longer use it. I just hope it won't be yet another subtle difference in behaviour between gtk and Qt apps.