Is the Ubuntu phone filesystem mountable in Linux via a usb connection?
Is there reason to believe this feature is here to stay? For me rather than full desktop/phone convergence, I'd be happy with being able to use my phone as the place I store all my essential data (as in a usb stick) and then use it on my phone or share it with my laptop whenever I need to.
Ubuntu Touch lets you access the file system via the MTP protocol, but does not support USB Mass Storage mode. In this respect, it is similar to newer Android devices and is unlikely to change. Out of the box, you should be able to access files on the phone using any GVFS aware application.
There is a good reason why many smart devices have been moving away from mass storage mode as a way of performing file transfer: mass storage mode involves exposing a block device, while MTP exposes a file system.
Unless a file system is specially designed, only one OS should access the underlying block device at a time. FAT is not designed for this use case, which is why older Android phones that supported mass storage mode would unmount the SD card before making it available over USB.
In contrast, with MTP the phone is the only thing accessing the underlying block device, and the connected computer instead makes file system level requests over USB (e.g. list directory, download or upload a file, etc). This means you can continue to use the device while it is connected.
