It doesn't completely block users from your site. It just makes them extra aware that any middleman will be able to see what they are reading, see anything they

It doesn't completely block users from your site. It just makes them extra aware that any middleman will be able to see what they are reading, see anything they send and could possibly modify the traffic. If you are concerned about the power-users who enable this being aware of these facts then maybe you do need https after all. (Of course I suspect this mode will be made the default at some point, but that is probably at least a couple of years off.)