If watch folders are very important to you you might look at Hazel (http://www.noodlesoft.com/hazel.php), which is like a more tolerable implementation of Folder Actions. It has a "Import into iTunes" action built in, and can be set to recurse subfolders. If iTunes is set to copy files on import, Hazel can be told to trash the original.

I wouldn't recommend it for just that, but even if it doesn't strike your fancy--after the trial expires, it still works on one folder with two rules (e.g. Add to iTunes & Move to Trash), so it doesn't really cost anything if that's all you use it for.

I'm not excusing the lack of a similar feature in iTunes, but that's what I use and I'm happy with it.