I would personally rather have the BD available as soon as possible. If that were some months from now rather than Christmas it wouldn't bother me, but I would much prefer that rather than waiting years for the trilogy to be complete and released as BD-ready files. I could always play the MKV from a thumbdrive, as you suggested, but there's something to be said for having it as a BD with menus and everything. You could always tweak the bonus content on the first two films and release a more cohesive BD set when everything is finished, but still release them as BD-ready DL's after you finish each one.