I've been a Spotify subscriber for 4 or 5 years now, and the three device limit for offline sync has always seemed a bit on the low side. At the moment I regularly use 5 devices (Android smartphone, android tablet, iPad Air, Linux laptop and Windows desktop). Only being able to use three of these for offline access is somewhat annoying (note that Apple iTunes allows 5 devices)
This means lots of resyncing and downloading of tracks over mobile data at times (I can easily chew through 700MB+ when I'm travelling - notably when downloading tracks before I end up at a hotel with terrible or heavily filtered/restricted internet or being in an area without 3G).
So, please up the limit for number of offline devices. I'm thinking 6 sounds like a more sensible number.
You might also want to think about providing users with other options for offline access, where they can choose the most suitable for their use. For example, you could have, say, three options where the user can change between them:
"Up to 3 devices, playback is locked out after 30 days of that device not being logged-in to spotify" (default) OR
"Up to 6 devices, playback is locked out after 15 days of that device not being logged-in to spotify" OR
"An unlimited number of devices, but you must have logged in within the past 48 hours for offline playback to function.")
"Ultimately, the reason we have 3 devices (3,333 tracks on each device) is down to licensing agreements. My guess is that rigidities in agreements such as these are unlikely to change anytime soon. Even in situations where mostly all parties agree something is both popular and practical (i.e. family subscriptions), things can take a long time to change if licensing discussions are involved. Although the idea is great since oftentimes folks have a work computer, a home computer, a phone, and nowadays, a tablet. Four devices. Please post feedback here."