Recently, on the blog
Answer: When they're an overseas design sweatshop using dodgy SEO tactics.
I'm used to getting comment-spam on my blogs. Disqus is cleans out most of it automatically, but a few make it through to publication. Fortunately, Disqus sends out notifications on new comments, so I'm able to manually filter out anything that doesn't seem genuine.
What is comment spam?
Comment spam is used by dodgy SEO businesses trying to build backlinks for themselves or their clients. The idea is that you choose a blog that's relevant to the search terms you want to build PageRank for and insert a comment (genuine interaction seems to be unneccessary) that has a backlink to your site.
While this can build pagerank on the site you've linked to, it only works if the site you're spamming has a higher pagerank than your own and is relevant to the kind of keywords you hope people will use to find your own site.
It also won't help you if you get branded as a spammer by doing so.