Primarily, I don't understand the purpose. I don't know a single person in the world whose musical taste I trust, or that I think is similar to mine. I've seen last.fm, facebook, twitter, etc., and they've taught me to ignore musical recommendations.
Malcolm Gladwell wrote a New Yorker article about how social network links have so little value because they're so easy. I might get 50 musical recommendations on facebook this week, and I'd only have to click twice to listen, and I'm not going to listen to any of them. But when the guy who lives next door hears me listening to Brahms, and stops in and says "You might like this other thing too!", and brings me a CD, I'll put it in my stereo and listen to it 2 or 3 times.
Second, the interesting music I'd want to share is largely not on iTunes. It's the bootleg of some Asian singer whose recording you can't get here yet (and maybe never). It's the Youtube video of some guy you kind of know playing at a club downtown last week. It's the out-of-print-for-40-years LP on my shelf of an incredible and rare recording of a popular group, that I got at a garage sale once for $4. It's the MP3 preview that some up-and-coming composer dropped on his blog to show you what he's doing.
Like everything Apple seems to do these days, Ping is probably great if you only care about recent mainstream name-brand consumer goods. (Lady Gaga at the announcement!) From a couple of longhairs who started out selling devices to let you make phone calls without paying, Apple has really turned into a company that doesn't care for the fringe -- even the legal fringe.
Third, it's part of iTunes. Back in 2003, I thought iTunes was awesome. These days, it's got so much crap in it, I don't even know how to do all the things I used to know how to do. It's confusing and frustrating and I use it as little as possible -- only for feeding my iPod, for things that can't be loaded directly. (iPods have gotten a lot more complex and confusing since 2003, too, but there's pretty much no competition in this space.) So Ping would have to be pretty awesome for me to want to use iTunes more than once every couple weeks, and it's not.
So if I wanted to do something that I don't, but can do already in better ways, but now do it in an app I hate instead of on the web ... I guess Ping would be for me.
You know what would be a better thing to put in iTunes, since apparently it's no longer just about "tunes"? Recipes! I need to keep track of them somewhere, and iTunes might be better than a bunch of text files. Plus, I actually do trust food recommendations from friends, so Ping-for-Recipes would actually be worth something to me.