There's no benefit to it only being able to reach some small group of people. There is benefit in being able to include only a small group of people in a particular conversation, but that does not require no interoperability, only a concept of groups and permissions and access controls. In fact, it is completely orthogonal to the lack of interop.
>There's no benefit to it only being able to reach some small group of people.
If something can just be locked to only talk inside the intranet/VPN it's better from something that can talk to arbitrary people the world over and is only configured not to via its own groups and permissions inside.
I thought we were talking about Slack, and several other hosted services, which don't fit the description you've just given, at all. But, for a self-hosted chat service, then yes...that's a benefit.
