One of the most serious problems with any blogging system, including WordPress, is that the comments area is wide open to that scourge of the Internet, spammers. In this case, it’s comment spam.
Comment spam is created by people seeking to boost their Google rankings by having lots of links pointing to their own websites. This causes a wide variety of problems:
When Google detects content spam, they will often block the site it’s coming from because it messes up their ranking system.
It takes up your valuable time and bandwidth to eliminate these posts.
If the onslaught of spam is heavy enough, it may result in a denial-of-service attack, intended or not, which is a situation in which the server tries so hard to post bad information and/or deliver notification emails to you that it denies service to the legitimate requests. In at least one case, a blogger received over two thousand email notifications of comments that needed approval; as he dealt with these, he continued getting more, ultimately crashing his mail server.
As you can see, even if you have your comments set to post only upon approval, this can be a serious problem....