My local "she of enormous complexity" has discovered the Windows Libraries in Win7 and everything on her computer ends up in the Libraries. It can be incredibly difficult to NOT DISCOVER that all your stuff is going into "libraries" whether you've intended them to be there or not, since Windows creates and maintains them by default.

The main difficulty arises when I try to back up her files for her, since everything that's in a "library" may also be in a folder that she created elsewhere, and the "duplicate copies" in backups almost outnumber the good files.

"Moving" a file to a library doesn't necessarily physically change the location of the information on the drive, but puts a "pointer to the physical location" with the same file name in the library, so the same physically stored file can be on the drive with pointers to it from more (sometimes many more) locations in the "folder structure" - or multiple copies of the same physical file can be physically recorded on the drive, consuming (sometimes large amounts of) unnecessary drive space.

With sufficient effort one can (according to Microsoft) figure out when deleting a "file" from a library only deletes the pointer to where the file is in another folder or when deleting in the library deletes the file itself from all places, but it does require some study. "She" loses lots of files because of the vagaries of that kind. (Of course it's my fault because I'm supposed to have them all backed up.)

Having the libraries automatically in a "user space" under the username of one operator also complicates accessing the files from a different computer being run by a different user in order to put them on an external drive temporarily attached to another one of the three computers it's otherwise convenient to use.

The Windows Libraries can be used by some people usefully. My impression from the instructions and pseudo-explanations provided by Microsoft is that the way I use my computers they're potentially hazardous for me. The dangers could be mitigated with some study of what they are and with detailed understanding of the nuances of how they work, but for my use they're unnecessary so I just ignore them. "Normal" users might actually like them, but I'm underwhelmed with the whole idea.

Microsoft has instructions for "completely" removing the libraries. The recommended procedure more or less just "turns them off" and they can be restored. Full "turn off" requires changing large numbers of Registry entries, so use of a "script" (call it a program) to make the changes is recommended. The removal programs are available from many places on the web, and appear to be the same everywhere; but so far as I've seen they're not available directly from Microsoft.

Since some people do find them useful(?) it probably is a good idea for most people to look at whether they find value in them before deleting, and that means you should try to figure out how they work before just blasting them away. Having them on the computer doesn't prevent you from putting files in a folder/file tree elsewhere.

You can make folders in a library and create/delete separate "Libraries" beyond the defaults, so for a single-user having all your folders as subfolders of "Libraries" isn't a lot different in actual use than having all your folders as subfolders of a drive, other than the possibility of having Microsoft "helpfully" rearranging your stuff "in certain ways."

To the original question, if you can select multiple folders to include in a play list, I see no obvious reason why you shouldn't be able to select multiple libraries into the list for the same kind of random playlist, since "library" (in this case) is just another filename that happens to be a folder with a lot of "stuff" in it (but perhaps a folder with slightly nefarious intentions about manipulating your files without asking for your permission?).

John