Yes, even if communities have separate relays, there is an inborn incentive to keep the events compatible with other communities and individual npubs.

I am also wondering if you aren't secretly fiddling away on a SupercommunityApp, that could be a client for multliple relays. 🤔

If I'm in the Republican Community and the Bible Study community, I might want to post something to both or move/share an event like a book between them. That is legit community-interaction.

Communities don't need to be like Fort Knox. They should be club houses. You can walk from one to the other, and take an interesting post with you.

You could always add encryption as an option, especially for a Private Chatroom, but they should have a use cases without it.