Indeed. You seem to assume that the music library is somehow static which isn't the reality for a lot of users of Spotify and other streaming services. There are at least a dozen albums released and reissued every day, and some people (like me) enjoy checking out new music on a daily basis.

It is simply not viable to keep buying all those albums and store them in a library, particularly if you won't listen to them ever again after the first listen. While an edge case, the shades of gray in the middle still demand a constantly changing library. Thus, Spotify and Grooveshark, whose catalogue I personally like a lot more, are much better choices than the 'online library' services.