With a cache of a spotify stream, for instance, will it be possible to tell how much music you have cached up in reserve? What happens when the next hour+ leg of your journey is through the countryside with bairly a 2g GSM signal, let alone a 3G signal, and your backup cache runs out?
Sod that. Plus, that's not even considering the total inefficiency of streaming the same content over and over again, why not just download it once?
To answer the article's question: I can't see myself streaming music to my portable player anytime soon. Capacity is high enough to be plenty for a trip of pretty much any length, storage is cheap, and mobile internet access is patchy at best. Whilst on the move, I'll take my locally stored, decent bit-rate, painstakingly tagged audio, cheers.
Stream to find.
Once found, buy, download, store, archive. Then use as you wish. (You could even do it in that order, if you were so inclined)