It would be nice to have clients that could merge several communities for convenience, but ultimately I don't think this is that big of a problem. People will generally gravitate towards the largest community and that will take over as the default. And even on Reddit there would still be splinter communities for people who didn't like the direction of the main community, sometimes even overtaking the old subreddit

No.

There's no need for monoculture - especially on a platform that aims to be decentralised.

Look at it this way - if some town has three gay bars, three metal pubs and three old-man real-ale pubs, it's not an issue for the LGBT community, the metal community or the drunk old men; they're just different places to drink. Possibly the drunk gay old metal fans might get confused, but they sound awesome and are likely welcome everywhere.

...actually, thinking about it, I bet the drunk old men probably do have an issue with all eight of the other pubs they don't go to. But that's just them.