Rating was mapped with the ID3v2 POPM tag some time ago. This makes it compatible with WMP and some other programs. MC12 can import the old proprietary comment frame format, but it writes a POPM frame on tag change.
Edit
At least it works for me. Though, I noticed that the format info shows "<unknown data type>" for the POPM tag. This is a minor bug, but it may cause confusion.
I can't find anything wrong with the Comment tag. Some other taggers may destroy MC's multi-line proprietary comments (COMM tags). Did you experience this or something else?