[estel]

Presumed pro: Spotify said at f8 that users who engaged socially with Spotify through their existing facebook functionality bother listened to more music and were much (I think they said 2x) more likely to pay for a premium membership.

If a Facebook account is mandatory (at the moment it isn't), they might expect a greater number of users to link their accounts. It doesn't seem unreasonable to expect that the number of people who additionally link their accounts will exceed the number of subscriptions lost from people who refuse to have a Facebook accounts. If a greater number of people linking accounts => increased Premium registrations, then this could very easily be a purely business decision.

[VonLipwig]

A better business decision would be to use the standard Facebook - or - Default sign up page. This will undoubtedly increase social usage by the Facebook faithful without alienating other users.

[StrawberryFrog]

Correlation is not causation, so forcing all new users into that workflow won't necessarily make them the kind of people who would do that voluntarily.

Also, this is a statistical relationship - there will be users who were likely to buy premium membership but don't like facebook, who are now less likely to buy. Or who are now reconsidering their existing spotify subscriptions.