Like I said, it probably depends on the usability. When a program bogs down due to too many items, it becomes unusable and the user will have to think of ways to keep the item count low.
I don't think people buy "Billy" book shelves for every room that they have only to separate books physically. If you can get to your book quickly, it won't matter if it's part of a gazillion or part of a miniature special selection.
But finally, I am just wondering what kind of motivation makes someone to attempt to use multiple libraries.
I know it can be done in iPhoto, iTunes and iWhatever ... all it takes is for the user to remember which button to press while clicking the icon. I always forget
I also don't have the need to separate my music or my photos into different kinds of libraries.
BTW, I like your Nav'i M&M Avatar