This new mode is not really focused on enhanced privacy and things like that, we are talking about saving some bandwidth in case of (normally) big channels, where seeing every single person for the users is not a necessary thing.
In combination with the m
mode it could be useful when it comes to XDCC channels, where there are hundreds of people you have no business with and you can't really chat in that room because of all the announcements scrolling... You came there to hunt the bots, not the people.
That, and as will said before - moderated interview with someone where hundreds can watch in the channel without being flooded with joins/parts/etc (I've had that little experience on Undernet once
I'm sure there can be more uses to this mode and it would help some networks to conserve bandwidth.