I have an use-case, but I doubt it's very widespread: I always carry my DSLR around, and wouldn't mind a few extra pounds in the bag.. so instead of a phone, I'd rather see a small
brick; not even meant to be held to your face ever, but used with a headset, and with plenty of USB ports on it.
I mean sure, phones will get as fast as desktops are today, but then a bigger computer will still be faster, and I would still have to do any heavy lifting on the desktop.
I don't do video myself, but even "just" processing photos and using Renoise requires more resources than I think even this thing can offer; basically, I have songs I can't work on unless the machine is at least as fast as my current desktop. The same would apply for doing stuff with WebGL: it doesn't have to be a gamer GPU, but it can't be a phone either. For me, that at best does for consuming, but not for creating. Especially since creating becomes more fun when it's more fluid, the minimum requirements don't cut it and more is better.
Even a mouse I have to lug around, and a foldable full keyboard, sound better than a laptop keyboard to me - I can't stand touchpads, or keys with 50 functions on them to save space. I could even get used to running around with a backpack if it meant I could have a real keyboard. And of course there's always internet cafes, which I would expect to offer a working spaces where you use your own computer, if stuff like this became reality.