Edit -- Seemed to have crossed post. This is for Rob --

Sure but if each tile is a map then all the one off unique images in that tile are in the file but each tile sharing the same images are still duplicated. Its convenient and can be useful but its not universally better than the way Dundjini does it.

Encoding binary data in XML would bloat it a bit - maybe add up to 35% but zipping the result would strip nearly all that back out.