There are probably loads of regular users of AB who never join. In particular, there's a crossword fan who frequently uses the computers in our local library to find the answers to crossword clues, simply by looking at this website and others. (I've looked over his shoulder plenty of times and seen him using this site but he's never been signed in).
Further, anyone who is considering joining a Q&A site is likely to want to see what it's like before joining. (I monitored this site for nearly a fortnight before plucking up the courage to join and to ask my first question).
So it's essential that what goes on here is publicly accessible!