Ponder this: what if the RPO has released an album with several orchestral versions of shitty 70s disco songs by a multitude of artists? Shouldn't the Album Artist hold RPO and the Artist tag hold the original artist?
No, the correct tag in that case would be RPO, since the RPO released it and performed all tracks.
A good example of album artist would be for this album, for instance:
http://www.discogs.com/Nightwish-The-Carpenter/release/675035Nightwish released the single, however, Thy Serpent and Children of Bodom also performed tracks on the release. Another (rarer) example would be if a "guest artist" performs a track on a release. In this case, the main artist would be both the album artist and track artist for the majority of the tracks, with the guest artist being the track artist for his and her track.
Another example, which is far more prevalent in my collection, would be the "doujin" (amateur, self-published) releases in Japan. In this case, albums/singles are "released"/credited by the "circle" (group that created the project), but the tracks themselves are credited to the individual persons who performed them.
Also, when you have an album with multiple performers, such as a compilation or soundtrack, it is a common practice to tag the album artist as "Various Artists".