This whole line of thinking has creeped me out since the advent of Google Docs. Call me a dinosaur, but I still see my documents as MY DOCUMENTS and I don't like the notion of giving up my privacy in them by default.
We've been trained by a quarter century of PC use to think of our spreadsheets and word processing documents as private unless explicitly made public, not the other way around. Users that link their own documents to their own Web sites obviously want to share those documents. But if you send the URL of your document to a friend and that friend links your document to a crawled Web site such as a blog post, your document could now show up in front of all sorts of people, not just readers of your friend's blog.
Note that last part... and the sort of un-written portion which says that although you can "unpublish" there's no "DO NOT CRAWL" flag. ::shudder::
