I used to do this in iTunes. Would love to see it implemented here.
I have lots of "best of" playlists. Always named Artist + "best" (i.e. Beatles Best, Depeche Mode Best, etc). In iTunes I would have an "all best" smart playlist that combined every playlist that contained "best" in the title.
Can't do it with Autoplaylist as it only lets you choose playlist "is" or "is not". I could create one that manually adds all the playlists together (one condition for each playlist), but I have way too many to make this efficient. And it wouldn't automatically update if I create a new "best" playlist.
Would love to see the "playlists contains" functionality added.