It can be "wrong", as in the case of people who follow thousands in order to get followed back, but then don't actually pay any attention to all those that follow as they're filtering via Tweetdeck etc.
But filtering can simply be a case of being actually being *more* interested in what those you follow are saying. I know I don't want to miss out on tweets from any of my "real life" friends, so I filter them in Tweetdeck. Similarly, I have a filter to group all music industry people together, so I can refer to this when necessary (eg. work) or follow conversation on a topic as it develops, without getting distracted by people tweeting on other topics.
It can be "wrong", as in the case of people who follow thousands in order to get followed back, but then don't actually pay any attention to all those that follow as they're filtering via Tweetdeck etc.
But filtering can simply be a case of being actually being *more* interested in what those you follow are saying. I know I don't want to miss out on tweets from any of my "real life" friends, so I filter them in Tweetdeck. Similarly, I have a filter to group all music industry people together, so I can refer to this when necessary (eg. work) or follow conversation on a topic as it develops, without getting distracted by people tweeting on other topics.