I think that's under 'Frequently Rejected' 
they want to avoid realm-wide spam
Though it would be useful for 'custom msg groups' (like when the sender chose 'send to some')
"Reply all" assumes that it is a sub-group of the realm, don't you think? Would hardly be causing realm-wide spam.
If anything, NOT having it causes realm-wide messages, since if there are too many people I just send to realm - clicking all those check boxes makes me twitchy
Realm-wide spam is a bit of a different situation than it was 4 years ago. Like I always say, you can't comprehend the message traffic that 150 active people can generate unless you've been there. Now, with realms being so much smaller, I think most of us would welcome more realm-wide message traffic.
Had that same thought the other day.
