Another form of twitter spammers are those who create twitter accounts solely as a way to generate links back to their porn/wares/ponzi-scheme websites. They follow hundreds or thousands of people, a few even follow back either out of politeness or laziness, and in the process generate hundreds of links to their sites all across twitter's "Who's following you?" pages. Rather than simply not following them back, I would urge users to to 'Block' these spam-users from following you.