Fairly frequently, the contact form on my blog gets comments that look similar to this (each field represents a text box users can enter into the HTML form on the blog):


  Name: 'ceguvzori'
  Email: 'gwizwo@avbhdu.com'
  Website: 'QrSkUPWK'
  Comment: 

vaB5LN <a href="http://pepddqfgpcwe.com/">pepddqfgpcwe</a>, 
[url=http://hvyhfrijavkm.com/]hvyhfrijavkm[/url], 
[link=http://cwiolknjxdry.com/]cwiolknjxdry[/link], http://ubcxqsgqwtza.com/



I'd consider them to be spam, but the sites they link to don't exist, so they aren't helping SEO or spreading malicious links. Not even the email host, avbhdu.com, exists. What is the purpose of these comments?

They're probing your site. First,  whether the comment will be published. Second, note how they use several popular syntaxes for links - it's an attempt to check which of them will result in an actual HTML link. If your site lets those posts through, expect more spam, this time more malicious.