Am I That Desperate for Twitter Followers?

I have been trying to increase my twitter followers and have been going through lots and lots of users.  I try to choose readers and writers (since writers are also readers).  But once in a while I get someone following me who is obviously just selling something.  I mean we are all 'selling' something but these are the ones who are only selling. You know what I mean.  I don't follow back but twice now I've given in.  I mean one had over 25,000 followers and one had over 100,000 followers.  And they are following me. How can I resist that?  And it doesn't seen like they are dropping people.  They follow almost as many as are following them.  I mean, if they retweet me even once, that's a lot of publicity.
So what's your opinion on these twitterers.  Am I desperate or smart?  What do you do?

And can anyone explain how you are following 900 people, have a couple following you and have just 1 tweet?