We have had WHOIS privacy as an opt-in service since it became available. Why? Because it is an effective speed bump against automated harvesting of personal information for spamming purposes. This speed bump is also very effective against the assholes that harvest our WHOIS information from our customers, then send them domain renewal letters to try to con them into switching over to a new domain that charges a hell of a lot more than we do, and without any value added.
If you are a spammer trying to hide behind WHOIS privacy, you are wasting your time because the second that we get a spam complaint, the registrar will immediately suspend the domain.