its true that thats the way MM writes them, but there is no guarantee that the value for the rating will be one that MM wrote, just b/c the text string is the one MM uses for its email ID. see here:
i have yet to do proper WMP tests, but the problem could really exacerbate there, since i believe it uses half stars and further granularity.
mp3tag needs to acknowledge that the raw data for ratings frames should also be accessible, regardless of what app it thinks wrote the data.