The KDE plasma active interface is geared towards Mer as well.
Tizen I don't think is Mer right now, but the impression given that that they are working on merging.
The use case for all three of these is still quite a bit different that your desktop use, So it might make sense to break them out with other mobile platforms. It may be that there's never a year of the Linux desktop as the desktop is becoming increasing irrelevant to anyone except content producers. Event the SteamBox is being marketed as a console plus. That is a console, with a feature of being able to use as a general purpose computing device, that is if any of you kids are actually old enough to remember that sort of thing.