I'm referring to the default "Album" grouping, in which albums are displayed as... albums. Since the very point of the existence of the [Album Artist] tag is to specify what should be displayed as the artist for an album, it seems a perfectly obvious thing to... display the contents of [Album Artist] as the artist for the album.
If you take such a liberal interpretation of 'Artist' so as to include 'featured' artists, then you're going to have such problems. There's meaning to 'featuring' (and other terms applied to performers who are not 'members' of 'Artist') which is best retained by storing it in a separate field. If there's any need to combine them (e.g., to show all artist associated with a track), this can be done using an expression (e.g., an expression field [Artists]). This also provides more flexibility in how tracks are selected. A smartlist, for example, could select tracks where particular artists are in [Artist], [Featuring] or both.
If you insist on allowing a piece of software to try to automatically determine whether an album is 'multiple artists' or not, then you're going to have problems. It is simply not a problem that can correctly be solved algorithmically in all cases unless you're prepared to settle for a very simple, naive model of library curation. This is, however, orthogonal to the matter of [Album Artist] display.
Also, this is not my "liberal interpretation' of 'Artist'". This is the officially documented, defined tagging standard for WMA (including lossless) files. A track with multiple artists gets multiple [Artist] tags. There is no provision for shunting certain disfavoured artists into some slightly more ghetto tag. [Album Artist] exists to specify what artist should be displayed when that is not the same as the [Artist]s. It's a simple yet powerful system that allows you to be thorough in attributing performances and still keep the album information display short and sensible. The iTunes de facto tagging standard works the same way (mostly - iTunes makes you list multiple artists in a single tag, but that's not really important here), and let's be honest: that's quite the most widely adopted tagging scheme in the world. I'm not just making this stuff up.
Essentially what you're suggesting here is that if I want to play in Media Center's sandbox, I have to play by Media Center's rules. That's a defensible position, but it's not one that I care for - and it's not what I hoped I was buying into when I purchased the product. There is a well-established, documented tagging scheme for WMA, and I use it. I'm more interested in interoperability - in fact, I simply require it, as my phone is not supported by MC - than in binding myself to one application's particular view of the world. There's a standard semantics for [Album Artist] supported by the vast lion's share of music player applications in the world, and it's a perfectly good and useful semantics. I honestly can't understand the resistance to using it. Leave [Album Artist] blank and get the existing artist summariser behaviour. Fill it in and get it displayed the way anyone accustomed to using [Album Artist] from elsewhere would expect. Best of both worlds. And if you
want to list auxiliary performers in a different field, nothing's stopping you. Everyone wins.
As for the other, orthogonal bit: personally, I couldn't care less about 'multiple artists' functionality, except insofar as it gets in my way. It doesn't really add any value for me. (Can you tell I don't listen to DJ mixes?) If I could turn it off altogether I'd shut up and go away. Windows Media Player and Zune simply have no such concept, and I'm fine with that. iTunes lets me explicitly tell it that an album is or isn't a compilation, and I'm fine with that too. Media Center pushes upon me its own educated-but-inevitably-sometimes-wrong guesses without any way to escape, and that annoys me.