The main issue why this has not been addressed in the proposed way, i.e., having both POPULARIMETER with the raw data in Email|Rating|Playcounter and the different app-specific RATING MM, RATING WMP, ...) fields is the following:
Displaying both fields, where in fact one of them is a derived field, i.e., would really break Mp3tag's way of showing what's in the file. You'd end up with a POPULARIMETER field (showing what's inside the tag) and a RATING MM field, which is then only a view on the very same data. Mp3tag is designed to only show one field per real tag data (that's either raw POPULARIMETER or a app-specific rating). Changing that would require per-field level sync mechanisms, where changing one field would also automagically change another — this is a direction I don't want to go into.
In fact, @MrSinatra I was thinking of the way Mp3tag handles rating lately and read some of your posts here and on the Winamp and MediaMonkey forums. My initial goal with adding the app-specific rating fields was to ease the manipulation of them and to provide means to display something star-alike without having to use endless scripting that splits up the POPULARIMETER values. Those fields were added when MediaMonkey used completely different values than the other apps (you've analyzed this thoroughly).
I now think that adding those app-specific was a mistake — which is unfortunately not so easy to revert. My dream-solution is to remove the different app-specific RATING MM, RATING WMP, ... fields and to have Mp3tag show the raw data (POPULARIMETER, RATING, RATE, ...). To ease displaying this data, instead of having dedicated fields I can imagine having a scripting function, e.g., $rating(%field%) that converts the raw value in a star-like representation.