The majority of new user signups I receive on my Wordpress blog look dodgy, with @someblog.com, @somearticle.com, or @gmail.com addresses.
What do they gain by signing up as a new user?
Some are attempting to leave their links behind, they believe the quantity not quality approach will actual help increase SEO rankings.
The other reason would be affiliate marketing garbage links trying to make a penny.
The reason spammers are creating accounts most of the time is so they can comment regularly on your blog to gain a link to which ever site they are trying to promote. Also they are checking to see if you "allow new user registrations" which opens up a door of exploits they can try in order to inject SQL into your site to add backlinks without your knowledge.
They are running bots that are designed to go out and gather intelligence by targeting Google searches such as “powered by wordpress” & " inurl:wp-login.php. The bot then gives them an insight on to which websites would be open to attack.
Hope this helps!
Zach Browne http://zachbrowne.com