Say you spend 5 months at $10/month building a library of 500 songs. That's 100 songs a month for $10, not too bad.
You choose not to renew the subscription because you need the extra money for Christmas or something. What do you have to show for the $50 you just spent?
It's down the drain. You have zero music for $50, plus you get to rebuild your library elsewhere should you move to another service.
And once again, some of us don't have the bandwidth to use Spotify without paralyzing our home networks.
I would have to buy around 60 CDs per month. It is a good deal.
There's no time to listen to each more than once.