I hope that's how ubuntu devs plan it. One app for all sizes, so all default phone apps must have desktop interface. Does this mean in future we will have all new default apps for ubuntu desktop, made using ubuntu sdk? New filemanager, music app, gallery, etc. Now ubuntu doesn't have default calendar app, it's sad.

I hope too. It's will be greate if ubuntu choose this way for application(one application - many UI for every devices).

I really hope it will work like that.
To make things faster, the Ubuntu developers should not only allow Phone Apps to have a desktop interface but also the opposite. For instance they should enable apps such as Gimp, etc.... to build a mobile interface without writing again the code (just write again the interface).

This way Ubuntu would surpass all competitors by bringing the powerful and efficient apps of the desktop to the Tablet and Mobile. We have to use the advantage of having a convergent OS for making easy to bring the power of the "heavy" apps to the Mobile by only changing the UI code and keeping all the technology behind.
This way we'd have rapidly a lot of quality and powerful apps for Ubuntu Phone.

When you look and Windows 8, a lot of people were disappointed because they couldn't install their regular PC apps on their tablets, and so Windows lost his only advantage which was the big ecosystem of PC apps. Windows dev are writting everything from scratch again with metro apps.

I hope we'll not have to write everything again and that will rather bring the already existing apps & libraries to the phone and tablets. I'd prefer have the power of Gimp on my phone with a new interface rather than having to use on poor,oversimplified,written from scratch mobile photo editor.