To add another angle not mentioned: Something I'm not sure of but interested in finding out is if multiple communities allow for better curation than one single large one.

For example, imagine a huge sub like /r/pics. When browsing "new" on that sub, the content goes away and is refreshed with even newer content in practically the blink of an eye. Because sooo many people are posting all at once.

As a result, a lot of good content gets missed in the flood of everything, and you have to rely on time of day and luck to get your post recognized.

OTOH with duplicate communities, the content gets divided and conquered a little bit better. One userbase can browse new on one community, while another userbase can browse and curate content on a similar one. In the end, both communities content don't get drowned out by the massive volume.

Once a multireddit like feature comes out, users like you and me can identify and group these duplicate communities and be none the wiser browsing all of them at once.