I had believed that the data stored in Baloo would always be used regardless of what is stored in the format specific metadata. What is your idea ?

Same question.

Could you rephrase please? I don't fully understand your question.
For now, the idea is to fallback to the metadata stored in the files if xattr data is not available.
So in case no xattr rating is set through e.g. dolphin, Elisa will show a rating if one is found within the file specific metadata.
Supporting the ratings tag was requested by users.
Note that we can only resolve (if at all) the issues of having two sources for the same metadata if writing these tags is possible, which I'm slowly working towards

Sorry for my bad English.
My question is about the long term plan for music ratings in Elisa, Dolphin and other software using KFileMetaData.
My understanding was that you wanted to keep using the extended attributes.

Long term plan is to write to both fields once KFileMetaData supports writing the rating, though that also has the disadvantage that the modification time is changed.
In the meantime, Dolphin is currently only showing the xattr ratings, the file specific rating is hidden (like it is done with the comment).
Elisa will fall back to file specific information as an interim solution.
When writing is supported, the "only" issue is potentially conflicting rating data, which the applications have to solve. But hopefully this is only rarily the case.