So let's put aside all issues of compatibility between different CPU architectures - let's ignore the fact that you can't run x86 binaries on any ARM-based CPUs (which means, pretty much, no games, no proprietary software, unless the developers specifically port those applications)... let's even graciously assume that the ARM platform will become powerful enough to run those applications, and that the mobile smartphone GPUs will become powerful enough to support all desktop needs. Let's even ignore the fact that most ARM-applications, for now, are developed for one or two platforms - Android and iOS, neither of which can be run on Ubuntu, due to Canonical stupidly deciding not to implement Android-compatibility.
Even then, the question remains: why? You come home, and want to use your desktop, with the comfortable and ergonomic interface that the combination of a keyboard, a large screen and a mouse provides. You then have to hook up your smartphone to all of these things, by some kind of interface that combines HDMI and at least several counts of USB... and then, the piddling 64 GiB storage space of your smartphone gets used up, and you need more space. Add another peripheral, an external harddrive.
Wouldn't it at this point be easier to simply have a separate desktop computer which you can connect your phone to? That way you can even use both independently. After all, you already need the space for your monitor, keyboard and mouse, so a slim router-sized mini-desktop unit won't be much of an issue space-wise. The whole smartphone-as-desktop-computer seems like an ill-thought-out concept that serves no purpose and solves problems that no one has. No wonder it has never become popular.
If Ubuntu jumps off a bridge, will you do it too?At this point Ubuntu is the only one to give my needs a go. If they feel Mir is the way to go ... So be it.
Using phone to have a presentation - again runs to the same issue: if the presentation place already has to have a keyboard, mouse, monitor - why not also have the CPU? With miniature desktop kits and all-in-ones becoming more prominent, it's becoming easier to put the CPU in the monitor, keyboard or heck, why not the mouse. If the CPU can fit in a phone, it can fit into any of those. And you'd still have the option to plug in your phone and remote to it from the desktop.
Your arguments are good if you're arguing for having good, open remote/sync protocols between the desktop and phone, but having the phone act as the desktop CPU itself (which was the original argument) still makes no sense.
Most of what I wanted to say GreatEmerald already did for me however I wish you Ubuntu trolls luck, it's going to be an uphill battle I think.