Joe,
You may be interested to know that many (probably all) Spotify subscribers are having this problem of forgetting off-line tracks. It's been going on for months! For evidence of this, just do a search of the various forums (even Spotify's own forum).
On my iTouch I regularly lose 40 tracks at a time, which I then have to resync - a real pain if you have limited broadband allowance.
Many, probably all, users only subcribe because of the off-line facility.
It's bad enough paying a subscription for such a "flaky" application. What is worse is that Spotify customer service refuse to even acknowledge that there is a substantial defect in the applicaton, and make any commitment fix it. Instead they try to fob us all off with excuses blaming "iCloud" and "ongoing improvements to the content database".
You say you are News Editor with What Hi-Fi?. Perhaps you can use your position to highlight this problem. Spotify are taking subscriptions for a service which doesn't work, and worse, continuing to advertise for new subscribers.
Many users are, like me, are looking for an alternative subscription service in the UK. Spotify's poor customer service will not be forgotten.