Because you can put them in a "Waste of Space" circle.
I sincerely hope we can prevent some circles from appearing in the stream. This missing feature is the main reason why I still use Twitter. On Twitter I have the people I follow so I can get information, while I use google+ to see what my real friends want to share with me. Therefore, If I could hide the "Following" circle from my stream, I would spend less time on Twitter.