"When you can't control the dissemination of your contact information through the WHOIS database, you risk becoming a target for those who have less than the best of intentions. Spammers may infiltrate your business email system – causing problems for you and your customers. Telemarketers also have been known to scour the WHOIS database looking for phone numbers for their own phone lists." (http://www.networksolutions.com/education/you-need-private-registration/)

The funny part is how these services claim that it prevents spam. I don't know how it can? The e-mail address in the Whois is forwarded to the domain licensee. How does that prevent spam, where everyone uses spam filters now?