Actually it is, just think about how long the rom browser would take if it had to unzip all roms 100%! ace files cannot be handled the way pj64 is handling zip files afaik.
yeah ace support could be added fairly easily if someone could be arsed doing it, but wouldnt bz2 compression be better than ace?
or how bout a special compression algo designed specifically for roms - like dzip for q1demos