Also, if it relayed anything that you played, some unsavory results might be relayed (and if it only relayed Apple purchased tracks there would be complaints).
A lot of people are going to want to present a different music profile than their actual listening habits.
Social networks sell themselves to users based on what the users can share and display about themselves to their social network. The more that a user can shape his persona (by revealing different information) the more appealing a social network will be to users, a big reason why facebook is so popular is exactly for this reason: it allows you to shape your presence in a very varied way (this is why pages is a killer feature).
To describe ping, apple should have really said that its a social network for ITMS, not a social network for music.
Someone's profile:
http://www.amazon.com/gp/pdp/profile/A2KEKKJ9CAC2KC/
Communities home page:
http://www.amazon.com/gp/tagging/communities/directory
Music community home page:
http://www.amazon.com/communities/directory/music/ref=tag_td...
A people affinity page, in this case, cats:
http://www.amazon.com/tag/cats/people/ref=tag_cdp_bkt_icus
Agree with your point that for now "it's a social network for ITMS". Seems a better way of thinking about it, and dovetails with defaulting to off and using ITMS account (a can of worms).