What will happen if comments spam is left unchecked and undeleted? ProBlogger discussed about The Cost of Comment Spam which include any comment spam found on your blog might invite more spam, reputation of the blog with too much spam, dead links that may caused by spam as almost all comments and trackback spam will link back to their sites, and also the cost of spam that may affect search engine ranking and optimization.
Akismet can be used to help to stop comment spam, pingback spam and trackback spam from reaching your blog. When a new comment, trackback, or pingback is been posted to your blog, it is submitted to the Akismet web service which runs hundreds of tests on the comment to check if it’s a spam. If it is a spam, it will not be published, but will be saved in the database for 15 days for your verification or checking, just in case there is false positive (valid comment been labeled as spam). After 15 days, and spam will be automatically deleted by Akismet.
The best thing about Akismet is Akismet is the plugin of the type of forget after install. There is no maintenance, no upgrading, no hassle and without any blacklist or whitelist to maintain. You may only need to check if there is any false-positive by marking the valid comments as Not Spam, or false-negative by marking the spam as Spam in your blog. All these actions will help Akismet to fine tune and improve its spam catching and identification system in effectiveness.
There is more and more blog platforms and software that has implemented and adapted Akismet plug-in, including (of course) WordPress, Serendipity, Movable Type, PixelPost and with Akismet API to integrate to other software and scripts.
Update: Akismet is constantly been updated for new versions to fight every changing spamming methods.