I am starting to think that any new microblog should have federation support at this point. There's too many social networks; things are too fragmented. It's definitely nice having a small community of likeminded folks, but the paradox is that every community in its early stage wants *more* users until the point where old users reminisce of the good old days when the site had "personality", and then leave to another smaller community.
I disagree. Federation is one of the reasons Mastodon/Pleroma are unpleasant; it contributes to context collapse and makes it harder to compartmentalize across forums. If I'm on server X, I don't want to hear from anybody on servers Y and Z (or see their untagged loli porn or alt-reich memes). If I wanted to interact with people on servers Y and Z, I'd create accounts there.
Both Mastodon and Pleroma have features to only see content from within your server.
Are you talking about the Local feed? That doesn't solve the problem of trying to have a conversation with people on your instance only to have some troll from shitposter.club or neckbeard.xyz jump in. Mastodon provides public, unlisted, followers-only, and direct--but not local-only, and unlisted isn't an adequate substitute because unlisted toots are still accessible from the open web.
Ah yes, I see what you're saying. Although I think that's one of federation's greatest strengths -- I personally have not experienced trolls derailing conversations, but I reckon that can be an issue on any large enough community. Mastodon and Pleroma can definitely find better ways to help isolate users that want to be isolated.