I know this is a big one but: Not only is .ufo supported by other software like Glyphs or Robofont but also the only accepted format for submitting fonts to Google Fonts. So support for .ufo/.designspace export would Type3 realy make an option here. Since .ufo is human readable the feature of exporting and importing the format would make it a lot easier to fix bugs in font files or change and extend things at a font file that Type3 isn't capable to do at the moment. E.g. Moving and deleting glyphs (under some circumstances) or editing advanced metadata.