The opposite seems to be true. According to the project's homepage, "Plasma Active is intended for all types of tablets, smartphones and touch computing devices". Without knowing the exact history myself, I'd understand from this that the main focus for this type of UI is tablets.

The way it works (see also this video) may be unusual at first glance, but I guess it makes some sense for a mobile device. - "Activities" correspond to situations and/or topics that you use the tablet in/for. You switch between activities with the wheel-like thing on the right, and each activity has its own desktop.

So I'd personally probably set up a "Couch Surfing" activity which I'd use 80% of the time. It'd include bookmarks of my favorite websites on the desktop, probably shortcuts to mail, instant messaging and a notes application... DLNA or something, if available, to connect to my TV.

A second activity would be "On The Train", where I'd have less internet bookmarks (probably only those that work reasonably well with the slow and unreliable Wifi I have on the train). Id have folders with music and videos, though, and maybe a PDF of alternative connections, just in case everything goes wrong and I'll have to make decisions offline.

A third activity might be an application I'm currently writing. It'll be full of documentation, various versions of the project and the usual boring stuff you need to get it running.

What seems to be a nice idea in the Plasma Active UI, too, is its recommendation system (panel you draw in from the left). They claim that the system learns things from my usage patterns and will recommend things like "you want to call Michael now" - because I usually call him between 11 and 12 when I'm at the office and I am at the office right now and the time's right. No idea how smart this will really be, but it sounds nice.

Knowing these things and watching the videos made it clear for me how the UI works and what the ideas behind it are. It's certainly more complex than usual touch UIs, which will almost certainly prevent it from becoming too popular in a market that is dominated by users who struggle to master their iPads. But for those who prefer power over simplicity, it'll be just fine.