Personally, I like the idea of having redundant means of contact. An example of why is something that happened just a few months ago. I had a domain with GoDaddy that I inadvertently allowed to expire in December so I transferred it to namecheap to avoid paying the $80 premium GoDaddy would have charged me to renew. Normally, my domains point to a contact page, but after the domain expired, Godaddy switched my nameservers to theirs and pointed my domain to their parking page instead. When I moved the domain to namecheap, GoDaddy's nameservers were still on the record and I completely forgot to switch them back to mine so the domain was now pointing nowhere and I had no idea that that was the case. Long story short, in January I received an email from a buyer who pulled my whois info and I successfully closed a $7k sale as a result. If my privacy had been turned on, I'm not sure if the buyer would have still attempted to contact me and if he had, I'm not sure it would have forwarded properly.
This is not the only example of sales I've had because my whois was public. They don't come very often, but they do happen and I'd much rather deal with a few spam emails then potentially lose money.
