A friend of mine complained on Google+ and attracted the attention of a GNOME developer who said that (a) it would be put back, and (b) one of the reasons it was scheduled for removal was that the Wayland graphics server doesn't have the notion of separate "selection" and "clipboard" shared-data.

I imagine there's a lot of resistance at the idea of including decades-old X11 quirks into the fresh, modern UI era, and I can totally understand that. The best argument I've heard for supporting something like it is to consider it an interruptible drag-and-drop operation. That is to say, in every scenario where you can select something then drag and drop it into another place, or another application, you should be able to select that thing then middle-click on the destination for exactly the same effect. The best argument for having this behaviour (at least as an option) is that it's more accessible than traditional drag-and-drop for users with certain mobility impairments, and fully-able users can use it to do things they can't normally do with drag-and-drop, like shuffle through windows to find the intended target. It also piggybacks on a lot of infrastructure (format negotiation, etc.) that needs to exist for drag-and-drop anyway, and lastly it makes old-school users happy. :)