It's all about quality, not quantity. I'm surprised to see Apple jumping into social music when there's already services like last.fm which already excel and will be hard to dislodge.
As long as (Long-Term Increased iTunes Store Revenue due to Ping) > (Development and Maintenance Costs of Ping), I can't see why it's surprising. Perhaps you disagree that the above equation will be true for Apple?
Your link just got me on the free trial... gonna try it out on my droid and see if it's better than Pandora, and maybe I'll subscribe.
edit: bleh, $5 extra just for mobile? Nevermind.
Personally, I think the offline play feature makes it worth it. None of the other on-demand services seem to charge any less for mobile either, which makes me think that the labels have put a price floor on mobile access.
Oh I did forget about that... although I wish it had some sort of auto playlist feature that mixes in recommended music... basically I want it to be Pandora that lets me sometimes choose songs.