You know all those bands that are not available in Spotify? They will probably not be available for Google either (AC/DC, Led Zep, Pink Floyd, etc). So, you buy those CDs and upload them to Google Play Music.
Once you do that, your music collection will be bigger and better than what Spotify offers. I like Spotify, but if Google does this movement right and makes the streaming and personal libraries to look like one, then I will move to Google Music.
Or if Spotify adds a locker service, then I might stay with Spotify. Of course, it’s just speculation.