First you may want to know why anyone may follow someone to begin with. As for me if I see an interesting tweet about a subject I am interested in I may follow that person. Occasionally I follow people who have participated in a Twitter chat session with me thinking we may engage later. More often I will search on a subject, hashtag or Twitter list and follow people on that list. Sometimes I am simply doing research for Social Media Edge radio and don’t intend to ever have a full on, two way relationship.
Next you may want to know why it is such a big deal to unfollow those who do not follow you back. Again, as for me, I do follow accounts who do not follow me back. @Mashable comes to mind as does @Biz. On the other hand if @kvbuckley does not follow me back (no offense k) I may eventually unfollow – not trying to embarrass anyone just using a real world example. So why did I follow kv to begin with? Because we’re in related industries and the president of her company is a friend. That doesn’t necessarily mean we will ever engage and that’s not a bad thing. Everyone does not engage.
Then why not unfollow everyone with whom you do not engage? Great question with a simple answer: if someone follows me I do the courtesy of following them back (eventually). I do not automatically follow everyone who follows me – I follow back manually. For that reason I may only go through that list once every couple of weeks. So if someone follows me, I follow them back. If we never engage that’s okay but I don’t expect anyone to follow me unless I follow them.