If you have a lot of singles, or home-made compilations, you'll have plenty of artist entries "polluting" your library each with a single album and a single song. Putting them in various artists just move the problem and they start polluting your official compilations and soundtracks list with many albums with a single song in each.
Here is an elegant solution to this problem. Have a "Singles" section, similar to "Various artists", but inside, there's no tree, just a flat list with only Artist - Song. I'm pretty sure it would also help with other problems by making a place to put songs which don't quite fit in you collection, without having to mess with their tags.

Sounds like a good idea. Could you please file a wish at https://bugs.kde.org/enter_bug.cgi?product=amarok so it can be tracked? Thank you.

done. Bug 248130 as been submitted.