The WHOIS process and protocol isn't just some nerd thing that goes back a hundred years; it's a valuable public directory for savvy internet users to be able to identify who owns a given domain name. Spam and security investigators find it a valuable tool -- even if sometimes bad guys submit bogus details, commonality of information across domains allows them to paint a clearer picture of who is behind a bad act or how broad that bad act may be.
It also makes it easier to tell the good guys from the bad guys. Good guys using their domain for commercial purposes, like your typical online retailers, they're not going to hide ownership of their domain name. When you look up their domain name in WHOIS, it's going to clearly state what company owns that domain and provide contact information for that company. Thus, if that information is missing, it's a bit like a restaurant hiding their public licensing information from view. It makes you wonder if everything truly is on the up-and-up.
That's why anti-spam and security folks don't like it when a company masks their ownership information by using a domain privacy service. That's why the ANONWHOIS blacklist exists to allow receivers and filterers to tag or block mail from domains who don't openly and transparently share their ownership information via WHOIS.
That's also why, if your domain ownership information is masked, you can't submit your domain to the Network Abuse Clearinghouse Database at abuse.net. Run by John Levine, the database is a valuable tool for spam reporting services (like Spamcop) and end users to figure out where best to send notifications regarding spam and abuse issues. Plug in a domain name (like spamresource.com), and you'll get the database entry (or their best guess) for where you should email in your spam report.
The submission process is pretty straightforward. In response to your submission, the abuse.net robot will reply to you and explain whether or not your submission was accepted. If accepted, it'll be available for lookup by users of the contact database. If rejected, you'll receive an explanation as to why.
If the reason it's rejected is due to the domain ownership information being masked, that response will read something like this: "The registrant's identity is hidden via Network Solutions Private Domain Registration. Legitimate mailers do not hide their identity. Contact the domain's registrar and remove the domain hiding."
All savvy email senders should register with the abuse.net database. Making it easier for spam complaints to find their way back to the sender, their ESP or their ISP makes it faster and easier for the ESP/ISP to help identify spam and deliverability issues, helping them help their clients stay in the inbox and helping them identify and shut down bad things.
Missing out on the benefit of being registered with abuse.net is yet another reason why you shouldn't mask your ownership info in WHOIS.