I don't know why anyone would want to use an Ubuntu smartphone, excluding the obvious free software implications. It's guaranteed to be worse than virtually everything else on the market. And why would Canonical even want to get into this business. You're competing against Apple, Google, Microsoft and Blackberry. They cannot win.
2: Direct development. When your phone is your computer there is no hassle with remote debugging or simulators.
3: Familiar native environment. All the software I already run on my ubuntu laptop can be run on my phone. I don't have to learn new tools, make multiple versions of my software or manage communication between my phone and laptop.
All of these things are sort-of possible already but it's awkward and difficult and you have to make lots of compromises. Having an easily programmable, always-on device in your pocket with the full power of a laptop and access to new emerging interaction hardware - that's going to be something new.