In the light of growing popularity of Linux handhelds that has desktop mode but uses a tablet form factor at their core using Plasma mobile on their built in touch screens would make a lot more sense than using desktop Plasma. However, when people use their dock to use that device as a desktop, it makes sense to have the full Plasma desktop. For now it is only possible to do that by logging of and on again to be able to switch between desktops since Plasma mobile acts as a separated desktop enviroment makint it very inconvenient to use it. (Not to mention that SteamOS for example is being an immutable system already makes it struggle to use it in the first place, but that one could be solved easily by convincing Valve to add Plasma mobile to to SteamOS or simply replace the OS with a different one without these restrictions.)
So there should be some solution to make it possible to use Plasma mobile on the built in screen while use Plasma desktop on the external display if the device uses it.
It wouls also be very benefitical for other devices too, phones, tablets, 2in1s.
For now though we’re not super focused on convergence related features because we first want Plasma Mobile to be a solid, stable mobile experience so more than just the most dedicated enthusiasts can actually daily drive it (and we’re getting there!). If you want to change/accelerate that you’re more than welcome to join in on the fun!