Is that a lot different from what happens now? Right now if you check the "Cache during playback", you'll get the recent songs saved. Also anything that's pinned is always saved. What goes out of cache is a bit magic, but it's unlikely to be the very recent stuff.
I think the difference that I would like is to have the offline cache on the phone sync to my playlist history across devices. So, if I'd been listening on the web client, or another device, then I'd like my phone to download all those recently-played songs and just have them available.
