1) allows for a lot easier migration away from gallery for those who'd like to switch to fotobilder
2) xml export/import can be a paid thingy on picpix.com
3) allows for scripts to be written that can do mass-manipulation/big-import/whatever using the lang's existing xml support
the only real initial hurdle I could see if that doing the raw binary data inline would make for some huge xml files, so you'd probably want to either put them in sep. files, still encoded (and probably use something like XLink) or just leave the actual files as "raw" filesystem files and put the filenames in the xml (so the xml becomes just metadata, which could be a very good thing) with the filenames being semi-random and all in the "source" directory (since keeping around a dir hierarchy would really be metadata about nesting, and we want that handled with the xml file).
With the core/base coming together so nicely, it seems like the right time to start with the nice-to-have's like this :)