I believe in freedom of speech and will normally allow good and bad comments alike to appear on my blog as long as they are not offensive or abusive. However there are automated computers out there just go around from one site to the next looking to spew spam into the comments of your blog in the hopes of creating links back to the owners website so that search engines will think that their site is popular.
While Akismet will help get rid of some of that junk, unfortunately the rest will get through. One way to deal with this is to enable moderation of comments. This basically gives you the opportunity to review the comments and only become visible to the public upon your approval. Sure it might be a little more work but WordPress can notify you by email when a new comment is added and give you the opportunity to quickly approve it or review it in more detail. It can even be set up to automatically post comments from people whose comments were previously approved.
Visit the WordPress support site to learn how to set up comments and notification emails in WordPress.