One good thing about multiple communities based on instances is you can have regional communities based on what instance they are on. /c/politics on lemmy.ml is technically neutral, but we all know its meant to be American politics, so opening a /c/politics on lemmy.ca even though it has the same name would serve a different purpose and be centered around Canadian politics instead.

Overall though I definitely think a multireddit type solution needs to be created for this, that way similar communities could be grouped together and mass subscribed to all at once.