In my own case, the attraction of Ubuntu Touch is to actually ship an entire semi-custom device with custom software for vertical use cases, mainly in B2B. Different order of magnitude of revenue / profit margin, with much smaller unit sales. I don't think Ubuntu Touch stands a chance with consumers at least for the next couple of years.
For people who don't know, these numbers are very, very small. I wrote an app store dice puzzle game on Android no one here has heard of and it has 20x those numbers.
As a developer, you have to decide if the decreased competition on the platform is worth the smaller user base when you decide if you will write a version for that platform. So platform size numbers like these are important.