As an example: If you have an account on kbin.social you can follow both !fediverse@lemmy.world and !fediverse@lemmy.ml, but these communities stay separate. This often leads to duplicate posts, and splintered communities. What NodeBB and Discourse have done is equivalent to if !fediverse@lemmy.world and !fediverse@lemmy.ml could follow each other, so a post in one of the communities would show up in the other community.
That's a neat feature! I think community aggregation in this sort of way is a positive and it could be useful for lemmy as well.
I'm not sure. Similar communities at different instances can have very different rules and vibes. There's a reason people prefer talking politics on Beehaw versus Hexbear.
I like the Kbin solution so far. Leave communities separate but cross-link and deduplicate individual threads from multiple communities in your feed. The implementation at Kbin is still a bit flawed, but the idea is sound.
The value is in the granular way that you can connect communities. You're totally right that there are a lot of cases where there are good reasons not to connect communities. That goes across instance borders (like you said, Beehaw and Hexbear would preferably not connect communities), but even for instances that are similar, not all communities need to be connected. In the current example of the Social Hub forum and the NodeBB development forum, only 2 communities (categories) are connected, and the rest is not.
I think it's useful if it's a feature you can choose to activate or deactivate yourself on specific groups of communities, but it sounds awful if the decision is made for you.