Personally, I've never bothered messing with whois privacy. If the registrar does it by default, fine, if not and it's public, that's fine too.
Key considerations for my own personal choice are:
- I have nothing to hide.
- It's easier to prove ownership with it public.
- It's easier to get interested contacts when it's public.
- There's an inherent positive trust factor when it's public.
- if a spammer really wants to spam you, they'll just get your email from somewhere else.
- If a stalker really wants to stalk you, they'll get your address from somewhere else.
- If you are breaking the law online, Big Brother can still find you, even with Privacy on

I can also understand why some may want more privacy (Movie stars, politicians, musicians, and other famous people). But, for the average joe, like me, I don't see the point flipping the switch.
I think most registrars offered privacy by default for a while, so most my domains are probably behind that default curtain, though, there are a few that aren't and that's fine too.