"If listener wants to have a Spotify party, with no ads, but with unlimited music, then why not allow them to buy (say) 6 hours of unlimited usage? (Buying tracks for a party is silly as you may not like all of them.)"
I believe you can already do this for 99p for 24 hours (or thereabouts).
Personally I think spotify is fantastic because I get to listen to what I want to and and I don't have to pay for quite so many MP3 downloads for my daughter (I don't mind paying for music but you can only pay for Miley Cyrus downloads so many times before the boys at The Pirate Bay become very tempting).
The full subscription is more than I'm prepared to pay for what is still a stream that restircts me to listening to the material on my PC, especially when the free offering is so good. I'd seriosly consider subscribing if it was the only way to use the service though, and would probably subscribe now if it was about half the price.
I've never seen how it's likely to turn a profit though so I've been working on the assumption, though hopefully wrongly, that it won't be around for all that long.