I use set subtitle all the time and it works just fine. It's great for classical music and box sets especially, but I also use it to identify bonus tracks at the end of an album and various other things. To be clear, there is absolutely nothing wrong with MusicBee and it's handling for these different scenarios, but for certain situations I really like the idea of the multiple, stackable panels. The artwork issue is the main advantage to it, of course, but there are some others as well. Currently I add the other artwork, the thumbnails of which appear at the top right corner of the expanded panel. You can click on them to view them. It works great. But for me this would be better.
It would also be helpful with a few minor, nitpicky sort of things. For example, sometimes some of the tags for the second disc or part are different from those of the first, and if you have those tags displayed in one of the header fields of the expanded panel, then one of two things will happen. Some fields will only display what is tagged in track 1, while others, like "artist: performer," will simply display nothing if the values aren't identical for all tracks within the album. The fields displayed do no aggregate the sum total of different values within all of the tracks in the album, nor should they as you would get some unwieldy results in many cases. Obviously, you can display these things just fine in the track information panel, but you only see them if you click on a track.
As an example, take "In Your Honor" by the Foo Fighters. It's a double album where the first part is electric and the second part acoustic. The way it works now, if you tag the genre for the first half as Hard Rock or Alternative Rock or whatever, and the second half as Acoustic Rock, if you have the genre tag displayed in one of the header fields it will only show Hard Rock since that is what the first track is tagged as. You could tag the entire album as both Hard Rock and Acoustic Rock, but that isn't ideal. If you had separate panels for each part, and it was implemented the way I'm hoping for, then the fields displayed in the second panel would be based on the tags in those tracks, so Genre would display Acoustic Rock in this instance. Again, it's nitpicking and silly, and only helpful in certain specific instances, but it would be helpful nonetheless.