Yes. Actually, when I posted that, I didn't realize one small detail.
The reason you can't multi-select Artists (or Genres) all over the place when you edit them directly is because for the entire history of MC, [Artist] and [Genre] were String-Type fields (single entry only style). This is, for many users (including me) a Very Good Thing. Otherwise it is a pain to make simple changes to the [Artist] field.
Say, for example, you have some files accidentally tagged [Artist] == The Beetles and you want to change it to [Artist] == The Beatles (or whatever, just replacing the existing contents with something new/fixed). There is a long historical record of this working simply in MC. You can:
1. Just click on the box for the right entry in the Artist panes in a view (Pane Tagging).
2. Drag-drop the files from the right-hand file listing onto the proper Artist in the "populated tree" on the left (Tree Tagging). This is doubly convenient because you can apply multiple tags at once. For example, if you want to change the [Genre], [Artist], and [Album] for a bunch of files all at once, you can just expand the tree to show the proper "chain" (so long as one file in the library is properly tagged) and then drop the whole lot onto the Album icon in the tree, and it'll retag all three of those fields at once..
3. Or edit it inline in Details view or in the Tag Action Window.
List-Type views are more complicated. If you do #1 and #2 above, the effect is additive, instead of replacing the existing field contents. So, you'd end up with a file tagged with BOTH The Beetles and The Beatles. This isn't probably such a big deal with that example when the two items would likely sort together (so be visible at the same time), but it can be incredibly frustrating when you are trying to do something like change from "Jonathan Coulton" to "Coulton, Jonathan" and you just keep missing the unchecking of that extra box. It makes all retagging operations require multiple extra steps (and brain power to pay attention to what is there). It also makes Tree Tagging essentially completely useless (assuming the intention is to replace the existing tag, not add to it). There are other issues as well (if you use a non-escaped semicolon in a List-Type field it is converted to a new entry, which confuses people too), and it causes additional complications when you want to push tags to disk in a filesystem structure (via Rename, Move, and Copy Files)... Does it scatter junk all around in ugly-named "someguy; somegirl; that_dude" named folders?
That's the history. People often came here and complained and asked for MC to change so that [Artist] and [Genre] worked as List-Type Fields (like Keywords and other built-in fields where you can multi-select). The answer was always: Sorry, but no, it has always been that way, and many users prefer it this way for simplicity. If you want Multi-Selected Artists or Genres or anything else, it is a simple matter to make your own custom user field (say [Artists] or [Artist (Multiple]) or whatever) and just use that instead.
And on and on we went like this, until relatively recently (MC17) when they added the Automatic Metadata Lookup System for movies.
The problem there is that TMDB does provide multiple (list-type) Genres for movies, and for that instance, while I'd never want a music file in more than one [Genre] (because my system is structured that way), it is quite handy to have movies that fall into more than one category (both Animated and Family, for example). This is made doubly true in that many of TMDB's Genres are quite broad. They have both broad ones, and more narrow ones, and just give them all to you for each film. If it was kept as it was, MC would just pick "one of" the Genres provided by TMDB (early builds with this feature did just that, they just used whichever was first in the list). No good.
So, there were many epic discussions. I advocated mapping the TMDB Genre list to Keywords instead, which was already a List-Type field in MC. But, people "call" them Genres, not "keywords", and so that's not so good. And other people came up with more esoteric suggestions where it would have some kind of crazy priority list and all sorts of other crazy things. In the end, Matt just pushed a build that did this:
Changed both [Artist] and [Genre] to List-Type Fields. However, these are "special" List-Type fields that use the old, normal, String-Type "editing controls" throughout MC. If the field contains a list (semicolon delimited string), it will be displayed and treated as such throughout. However, when you try to edit the field, it will act like a String-Type field (replacing, not adding).
They threw [Artist] in there because it was a longstanding issue anyway, and this was a good way to shut "both of the camps" up.
The downside was that if you wanted to actually use [Artist] and [Genre] as List type fields with manual tagging (rather than auto-parsed stuff or expression results), you had to do it the "old manual way" by typing in a string with semicolons to separate list items.
It worked. It shut most of us up, and the system works beautifully for [Genres] for movies, and I (and my ilk) can ignore it for [Artist] and music (as I prefer).