This is news to me that WHMAP is such a big security risk. Considering I have port 25 set to localhost only. It's not an open relay or anything like that and I check logs for strange mail related stuff.
Now, suppose a spammer signed up on your servers. The fact that you have it set to localhost only, make no difference.
The only thing you have secured by setting it to localhost only, is that people that do not have a domain or an account on your server, cannot spoof your SMTP.
However, if you have a spammer on your server, they would be mailing from localhost, and the fact that they would use SMTP means, they could literally send out thousands of emails in less than a min's time, and as said...it would come from localhost. In fact, using that, they could effectively, and quickly forge all of the mails to make it appear as though it's coming from YOUR domain, if they chose to do so.
Just because it hasn't happened yet, does not mean you should leave vulnerabilities like this open.
All it takes is once.
For example, most people lock their doors at night, and have never had their houses broken into, yet they continue to lock their door at night, because they don't want to take chances with their family and home.