Do they think Ping's going to persuade me to pay over the odds for digital music and give up on CDs??
Unfortunate fact is, there are a lot of people out there (I'd say most of us, but to varying degrees) whose music taste is largely influenced by what they think it says about them to others. Music as a status symbol. And status symbols work even better when it's less apparent that you're actively trying to signal.
So if my stream updated automatically that I'd just listened to the new Arcade Fire album, problem solved. But if it doesn't, and I want to impress people by showing that I listen to it, and there's no way to do that on Ping except by buying it from iTunes... I think that would have an effect on a lot of people's judgement. Most of the time not enough to send you over the edge, but on an aggregate scale, often enough to bump up sales.
Again though, this is contingent on Ping taking off as a tool people actually use for music networking. If no one's looking, no one cares what they look like.
Actually, there is. In my opinion it is a very bad way to do so, but you browse on the ITMS to the music you want, click on the down arrow and click on Post. Then you can add a message and it will be appear on your Profile.
But as I said in my comment[1], they should have made all that accessible from within my library!