Follow for follow is pointless because you basically have robots which aren't interested in your content.. the only thing it does is increase your following count by 1 which is useless.
It's like having 500 fake friends on Facebook, nobody cares and it doesn't benefit you :)
That's also a GREAT point. Many of these accounts aren't even real. A bot can leave comments as well, so a lot of these follow for follow people are just bots. It's best to stay away from them.
In fact, since I've written this post, at least 3 more "follow for follow" users have followed me. Becuase of these people you can't trust your growth numbers as having any value on this site right now.