That a feature that i miss, maybe also the ability to chose the separator between each track artist if we choose to not use the index=1

That way when we use a file manager to copy some track to the cell phone it still possible to know witch artist on that track without opening the file.

Dan,
Yes that's what I see, except no commas or spaces between the multiple track artists. More like:
album name/track artist 1track artist 2track artist 3 - track 2.mp3
My collection uses the ID3 standard "/" for track artist separation, which works well with Windows Media Player/Center and XBMC.

You nailed album artist. I don't use it because my compilation naming would have "Soundtrack" or "Various" in it.

An index=1 in the <track artist> would be exactly what I'm looking for and would solve my issue.

Right now my naming looks like this:
<album artist> - <album name>/<discnumber>?|CD<discnumber>/|<tracknumber:nn> - <track artist> - <track name>

This would be great:
<album artist> - <album name>/<discnumber>?|CD<discnumber>/|<tracknumber:nn> - <track artist:index=1> - <track name>

Can I confirm what you are seeing? Something like:

album name/track artist 1 - track 1.flac
album name/track artist 1, track artist 2 - track 2.flac
album name/track artist 1, different track artist 2 - track 3.flac

And so on? You really just want:

album name/track artist 1 - track 1.flac
album name/track artist 1 - track 2.flac
album name/track artist 1 - track 3.flac

Can you explain why <album artist> cannot be used? You might get "Various" for many compilation albums. Is that why you don't want that?

Maybe this could be solved with a new parameter to the <track artist> token, configurable in custom file organisation patterns:

<track artist:index=1>

Meaning: the first track artist.

Or something like that...