Yeah, I've requested this before as well, don't know when/if it'll be implemented.
I think the biggest problem is figuring out how to make it work with files from different directories. I suppose you could have an option to move every other non-song in the same directory as the songs to their respective folders, but this presents other problems: what do you do when you have a folder with assorted songs that will be moved into different directories? Do you move the other files into the first directory, last directory, or neither? What if that folder happened to be an important directory full of system files?
These would be partial solutions to the afore-mentioned problems:
1. Add an option to choose how songs are differentiated using TAGZ. Default would be %album%. What this means is that if the Album tags are all the same in a folder, then move the other files into the new folder.
Problem: if you had a bunch of files from the same album that are not in their own folder for some reason (I've had this before, especially from direct downloads) then it's not going to prevent foobar from moving all your other files.
2. Add an option to choose the types of files foobar will move. This at least partially fixes the problem with moving system files and other junk.
Problem: Even when combined with #1, other files could get moved, especially any stray .jpg or .m3u files one might have lying around.
2b. Don't move any extra files if there's non-listed filetypes in the folder. Would fix most of these problems.
Problem: Won't get everything all the time.
3. Search all other files using TAGZ criteria.
Problems: Potentially slow in big directories. Won't get everything (ex: folder.jpg is really common)
4. Implement #1 but slap a WARNING-style message box on it before doing it, so users don't accidentally screw up stuff.
Problem: Message boxes are annoying, people might ignore them anyways

Options #2b and #4 are the best IMO, message boxes aren't that annoying (you could just put some big text next to the option anyways) and a basic file type filter would eliminate a lot of the system files from getting moved.