That’s an important and valid concern. What if the community federation could allow mods on your instance to ban users from other instances? You’d not see that user’s posts or comments when viewing a community from your instance. The downside is that your mods would have more work.

OP didn’t say force. OP specifically said allow.

This is a really good idea. Multi-instance communities would not just provide content redundancy, but also some load balancing. Each multi-instance community would become it’s own little CDN. Duplicating the data across instances does pose a problem of bloat, but I think the benefits outweigh the risks.

That system makes the instance a single-point-of-failure for the whole community, which has been a big problem lately. If communities could easily be multi-instance they would have redundancy. That seems like a good reason to me.