As you requested, it would be good if MC could preserve the existing unrecognized multi-line comments when it adds its own tags.
Yes, preserving existing information is essential. Especially the information stored in multi genre and image tags contain a huge amount of work and effort.
As I already mentioned, it would be great if MC would support multiple genres, but this is optional as long as all existing information is preserved.
So far the only option is to switch of tagging altogether. This is a workaround witch I can live with for a while.
Regarding the developers, I really appreciate the work they did on MC. It is the best media software I know of. I especially appreciate the fact that MC12 actually make use of the rating and play counter fields in the ID3v2 tags with virtually no other software does.
As minor as this issue may look like to you, it prevents people with a well tagged and maintained media library from using MC.