If a new user on my instance was able to subscribe to a comunity on another instance, and all the posts/comments/likes of that community was being fetched so it was browsable on my instance - I would see no issue.
But as of now, if a user wants to browse content from several communities on different instances - and discover new interesting comunities on different instances - while being a user on a instance filled with "like minded users, they would need a user on all instances which hosts communities of interest to 1) discover these communities 2) view the posts from these communities.
What you are describing are single entities of communities, which defeats the purpose of federation completely, in my opinion.