You will need to subscribe to the Apple service if you want to use your existing collection there. So that's not so far from the Spotify solution.
I listen to a lot of music that isn't on iTunes or Rhapsody or <insert service here>. Having the ability to access a more complete library of what I actually want to listen to presents a huge value to me. Hence my willingness to pay for a locker over a streaming service.
But believe me, I know not everyone is as picky as I am.
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.