Sure, but you have to be careful of the gradual lockdown effect as more people get involved. The handling of non-public information is a cross-cutting concern. If everyone builds their software and security models around the assumption that all posts are visible to everyone (because it's the common case, and they decided, just as you're saying, that supporting the common case was the most important thing), then going back later after everyone's gotten attached to the software and trying to add private multicast without any leaks can be a nightmare. No one will be able to use it because their friends won't be able to use it unless every piece of software in between makes it work.
I'm tempted to compare to how deploying new transport protocols over IP is nearly impossible for consumer clients now, because everyone's built NATs that assume TCP and UDP, because those were the common cases and therefore the important ones and now anything else is instantly hosed. It's a bit of a bad example, though, because in the case of transports there are other reasons as well.