Spotify is go.
Let’s generously assume that only 95% of the 800,000 are free: compared to those users being 9.99 paid subscribers, that’s a shortfall of just under 100 million euros annual subscription revenue that needs to be made up in advertising. I admit the comparison is a bit disingenuous given the explicit tactic of deploying a free tier to promote the service. But Spotify must be careful not to expect to convert the majority of those to paid subscriptions. They’ve cast their net among free music fans and they’ve reeled in a nice catch of freeloaders who love their music but predominately won’t pay to stream it. Charging for must-have added functionality, such as mobile, might be an avenue, but Last.FM, Pandora, imeem etc have all set the standard of mobile support being free. So charging for mobile would arguably be as damaging to their growth potential as if they’d only launched with a premium offering.
From Mark Mulligan's Music Industry blog.