Spotify for Windows Phone is an interesting value proposition. There's no doubt that Spotify has many fans and a strong user base, but many of them have been with the Swedish music platform since before Windows Phone 7. Music is a powerful motivator for someone buying a device, and the lack of a dedicated and worthwhile Windows Phone 8 client has likely scared away people looking at the platform.
It also means that many people who have made the jump to Windows Phone have already decided on an alternative music subscription service. MixRadio is one option, delivering a curated yet radio-like stream, while the Xbox Music pass opens up a huge catalogue for streaming and downloading direct to the Microsoft powered handsets.
Which leaves Spotify's update to the client in a bit of a no-man's land. Spotify fans aren't heavy Windows Phone users, and Windows Phone users are unlikely to be bigging up Spotify.
That's not to say Spotify will not have fans on Windows Phone - with the installed base of Microsoft's mobile platform there will be users out there, and perhaps that explains the sudden update to the app. To be honest, I think the better explanation is the idea of a universal app. With Windows 8 focusing on touch, and the new common developer environments, Spotify could be one of the first big unified apps, running on both Windows Phone 8 and Windows 8. At the moment it is not labelled as such, but the UI has enough indications that it could run nicely on the desktop/laptop platform.
The switch hasn't been thrown yet in the store, but the demand for a modern Spotify client on the desktop is huge. this is where Microsoft's dual strategy comes in to play - if someone can be convinced to code for Windows 8, it's a small step onto Windows Phone 8.
I'll be watching to see if Spotify does make the jump to an official client that supports Windows 8. If it does then I'll also be watching to see how much of it is the Windows Phone 8 client with a tweaked UI layer.
This is the future that Microsoft promised, let's see if one of the largest third party media apps feels like it is a path they can follow. If they do, the Windows 8 and the Windows Phone 8 community will both benefit.