I haven't tried it either, but I suspect you are right - it would be saved as `/foo/title` rather than `/foo/foo feat. bar - title`. 

The point about fragmentation is that if you have an album by foo and several non-album tracks by foo feat. different artists, then you would get:
```
/foo/duets/01 foo feat. bar - title
/foo/duets/02 foo feat. bell - title
/foo/duets/03 foo feat. star - title
/foo feat. dot/NAT title
/foo feat. dash/NAT title
/foo feat. asterix/NAT title
```

But really, the NAT tracks should all be in the foo directory since foo is the primary artist.

To get the artist into the filename for the NAT tracks, obviously you could do something like `$if($or(%_multiartist, $eq(%albumartist%,)),%artist% - )` in order to get `/foo/foo feat. bar - title`. 

But then the default renaming script is becoming somewhat complicated for a novice to understand. OTOH, if we are trying to make the default naming script as good as possible for a user who doesn't change it, and we reasonably expect users to know what they are doing if they do change it, perhaps the additional complexity is not really a problem.