MC always uses its internal tags for use in views, etc.
When you say 'internal tags', do you mean the JR library? If so, I understand that, but am having trouble working out where that library gets its information from.
I guess if I want to completely refresh the library I have to delete the path to my movie files, then reinstate it. Or else 'Update Library (from tags)' for all files, or the ones I want to change. Or wait for MC to notice the timestamp change.
Surely JR does use the sidecar files for tagging, if the relevant tags are not in the video file. I suspect the first port of call for information (if you have disabled 'Get Movie and TV info') are the file tags, and then the sidecar.
However video files are notoriously difficult to tag, especially formats such as Matroska, and editing incorrect tags can be very time-consuming.
OK, I accept that you cannot force MC to read the sidecar only. And from reading the forums, it seems some people see the sidecar as an ugly and inelegant method of tagging.
But this would be of benefit to people such as myself, who use an external app (from the ergonomic comfort of my computer desk) to tag files and create sidecars.
So just puttin' it out there that the option of using the sidecar exclusively would be useful for some punters.