Musicmatch stores the rating information as a COMM tag
So Does MC11
the tag comm name for MusicMatch i think is "MusicMatch_Raiting" (i forgot)
the comm tag MC11 uses is "Raiting"
I would think that is a better tag name since it is not branded with the "MusicMatch" Name
It does seem rather silly that ID3 has no standard rating tag, but that's not a situation that we can readily fix.
Comment fields are fine, as long as the software/hardware we use can be configured to properly interpret the information stored within them. I'm guessing, though I don't really know, that Musicmatch and Media Center ratings don't translate between the two pieces of software.
The thing I'd love to see, though I'm not sure whether or not it's possible, is the ability to link library fields to specific ID3 tags. Not the ones already standardized, but ones that don't have a standard or ones that are created by the user. Of course, I'm not a programmer, so I don't know just how complex that request would be.