Spotify was a great idea before the emergence of music lockers - if I wanted access to music on the go, I could use a streaming service. But now that I can use a locker service to stream my own music library, I'm far less likely to pay for a Spotify subscription.
Case in point - I would have instantly sprung for a Spotify subscription two or three years ago had it been available in the States, but now that I'm using Google Music (albeit for free for now), there is no incentive for me to switch to Spotify as anything more than a semi-casual user.