> Now as to whether people actually want this, I don't think they do.

They do. Surely you've seen statuses to the effect of "any one of my friends who supports [belief] can just go and unfriend me so I don't have to unfriend you". Certainly I have friends who are so stridently in support of things I think are unethical/illogical/foolish that I've unfollowed them because their posts make my head hurt. If it weren't for the bubble there'd be a lot more of that.