after some research i think you should use album artist only in compilation style albums.
even in the example i am using directly right now, where i have a collaboration between 2 artists and also featurings in it, if i leave the artist field for the album artist field for the 2 artists i will have artists tag empty if some of the tracks are anyway only made by those 2 artists and without a featuring.
so either all the tracks are made by different artists and you have a reason to trump these, or you must keep the 2 album artists in the artist field even if there are some feats.
that's how to use album artist tag i suppose. because also it is seems to be not an official, let's say, tag. it is of secondary importance and you should have artist tags first.