While filtering would be a good feature, I think saying its a killer feature is going a bit far. I don't think the reasoning that you could follow twice as many people would make much sense from a roi perspective. If your interested in roi, you wouldn't even worry about following people, you would create custom searches about topics you are interested in and deal with those. You would certainly get more info for your time spent on twitter that way and much more focused. You can do this with basic tools like Tweetdeck search columns or even or saved searches in the standard twitter web interface.