That's worth knowing, but in this case (and I may not have been clear), I was referring to Tags in General, going beyond just MC18. (Or at least, tagging for use in MC18 while keeping compatibility for non-MC18 use)
Also, a separator won't work in this case because I might want "Carlos Santana & John McLaughlin" to be sorted to both their lists, but I don't want "Derek & The Dominos" to be sorted as "Derek" and separately as "The Dominos" (for those who don't know, it is a fictional name, there was never anyone named Derek in this context).
So, I guess the point is that "Album Artist" could use to be a "List" rather than a "String" for the same reasons that "Artist" is a List.
I'm not sure whether or not MC does this sort of change to a common tag field, only in conjunction with general changes in the Tag standards, or if they sometimes do it unilaterally...