It's interesting because this topic was one of my early posts back in 2015 about MC21. The essence was the opposite -- to never tag videos. MC tags wmv files because, if I recall, it uses Microsoft's interface and tagging came for free. I was hoping even this would be removed.
For me inserting metadata directly into video files would be a nightmare scenario:
(1) Video files are large: If I choose to change the Series name for 150x 2GB episodes
(1a) This will take minutes? more? instead of seconds
(1b) It will likely mean I have a 300GB backup instead of 3MB of sidecars
Heaven help me if I choose to re-analyze audio.
(2) Anytime a file is edited there is chance of corruption. Do I try and play everything to spot-check that nothing went wrong (before the next backup)? If side-cars go bonkers I can refetch and rebuild metadata for a series in a hour so why risk corrupting media which I painstakingly ripped, encoded, and verified. I have always been a bit bemused that audiophiles go through monumental cost and effort to create beautiful playback environments yet seem comfortable with software modifying their pristine lossless media just to insert ID3 tags.
I don't know if anyone else has similar concerns. For my core library, I think side-cars are perfect -- per file, persistent, editable metadata external to the media.