I just read an article on ReadWriteWeb about people buying Twitter followers. What a waste.
Most of these people who are buying the followers are probably selling something and are hoping to leverage Twitter as an advertising/marketing platform. What they don't understand is, just because they are gaining follower, doesn't mean that any of those followers are listening (to your marketing message).
And listening not following is the key. You can easily follow someone on Twitter, but not listen to anything they have to say, which is what is happening with the purchased Twitter followers.
Speaking to 1 person and have that 1 person listen yields a net gain of 1 person listening.
Speaking to 20,000 people and not having anyone listen yields a net gain of 0.
Its about listening, not following.