It would do two things:
1. Compromise privacy - whilst it could be said that priv/+s rooms are the place for private chats I don't think it is fair for two people having a conversation thinking that no-one else can see it only to have people join and be able to view what was said.
2. It would place an un-necessary load on the server, especially on large networks where hundreds can join rooms every single second.
The idea has some merit but not enough to counteract the above issues, and possibly others. Also, this is a feature that would be better suggested to IRCd coders, rather than client coders as the primary function would have to be on the server first.
