This idea popped into my head this morning. I was on the forum to suggest it, and found you beat me to it. Clearly, I think it's a great idea! Though I thought I might add some cases for the feature.
Social networking is all about users connecting through content. Somebody generates interesting content, somebody else notices the content and reciprocates in some fashion. The Zune Social is sorely lacking in the user generated content area. Being able to blindly stumble upon other people with similar music interests is not enough! The Social need the Zune team to facilitate finding other people faster, easier, and more accurately. Playlists can do just that.
Playlists are an ideal solution because they provide highly accurate indexes of a person's musical preferences. By allowing users to search for playlists by artists or genre, you create many pointers to an individual-- greatly increasing the chances that people with similar interests cross paths.
Playlists also create significant opportunity to increase revenue by driving Zune Pass sales. Ideally, a user without a Zune Pass could listen to a whole playlist once-- or at least listen to extended previews of each track. After that they would need to have the tracks in their Collection to continue listening to the playlist. Even on 10 track playlists users will likely realize the value of subscribing to Zune Pass for a month to get the playlist plus all the other access that Zune Pass provides.
So help us out, Zune team! We need a more social Social!