Well, there are usability and historical reasons.
Up until last year, MC always treated [Artist] and [Genre] as regular string-type fields, not List type fields. People can disagree, but I (for one) appreciate this and wouldn't want it to be changed. That's because when I want to fix an Artist tag from, say, "Beetles" to "The Beatles" or "Tori Amos" to "Amos, Tori", I can just click one box in the Artist Pane to apply the "corrected" tag.
If I decide a song belongs in a different Genre? I just click a box.
Otherwise, it is difficult to clear tags, and confusing for LOTS of users. If [Artist] and [Genre] were regular List Type fields, they'd work like Keywords does in the panes. Meaning, if something comes in tagged as "Beetles" and you tag it "The Beatles" in the panes, but then don't remember to find and UNCHECK the "Beetles" box, now you have it tagged BOTH ways.
So, the issue was that MC was importing tags from TVDB and other sources that did use multiple Genres and Artists and whatnot.
So, they changed it, but only partially. It still works like a traditional string-type field as far as MC's UI is concerned (it IS a string-type field internally) but it is parsed for display as a List Type field (semicolon delimited). It was a compromise. Allow MC to use the imported Genre lists from TMDB and whatnot, but not break existing workflows for everyone.
It is an ok compromise, but I do find that sometimes I want to tag multiple Genres, and it would be nice to be able to do it in the Panes, if there was a way to do it without having [Genre] and [Artist] ALWAYS work as List Type fields.
I think allowing multiple entries by control-clicking tags in the Panes would be the simplest solution.