In my opinion, if a person cannot be bothered to join the site, then they should have a 'view' option only. The idea of WT, is to have ONE tree, as fully sourced as possible, with no duplicates, and with every member striving to this end. We sign the honour code for this reason. If an anonymous person is allowed edit access to records, then anything can happen, and the quality of information will then become suspect.
In a totally worse case senario, we could end up with Barack Obama, married to Hilary Clinton, and giving birth to William Shakespeare. (Joke)