And what if what you really wanted to share was a photo, or a comment, or an article from NYT or Vogue, and not a long piece of writing? Isn't that what most of Facebook is used for?
On Facebook, one at least has some protection against outsiders searching for their name and finding one's "silly" (careless) postings. If all posts were open, no such protection would exist. I think this could be a real barrier for widespread adoption of this approach.