one of the advantages of having huge libraries is that you can group up playlists worth 20-30hrs and play them, you could switch playlists when you get bored of the one you're currently listening to.
usually you end up similar to:
playlist1 = monday
playlist2 = tuesday
playlist3 = wednesday
etc.
and you can't do that with less than 10K library, that is if your playlist is at least 500songs large.
edit: on a note though, about 60% of the songs i have in stock were the kind of "tried it, didn't like it" but still remains on my harddrive.