Regardless of precise dates, why would someone bother with such a latecomer to the mobile OS game? There are actually a few good reasons. Most importantly, Ubuntu is so lightweight that — by the time Ubuntu 14.04 rolls around — it'll be able to use the same code across all four form factors, with the same security features, user profiles and UI fundamentals.
Since the OS will be a constant, a smartphone-oriented app will work on an Ubuntu tablet or any other Ubuntu device without having to be ported or even tweaked.
Microsoft and Apple both seem to be trying to reach the point where they have one operating system that runs on everything—phones, tablets, laptops, desk tops—but neither one is quite there yet. Tablets and phones don't yet have the computing power to do everything a desktop can do, so we have these two parallel operating systems where apps that work on mobile computing devices don't work on desktops and vice versa. But ultimately, all of our screens are going to be linked all the time, and they're going to do so effortlessly, without any patches or kludges or workarounds. This is the sort of thing that Apple or Microsoft will announce to great acclaim in four or five years, and when that day comes, someone should be there to point out that Ubuntu got there first.