I think you'll end up using a lot of storage space; in my totally uninformed understanding, you would end up storing the full contents of any community/sub that you follow.
Won't that happen anyway? A typical instance with 1000 users is likely to be subscribed to greater than 50% of the active communities out there.
Yes but the storage used by that instance is shared among 1000 users, rather than all devoted to just one user. Theoretically there would be a fair amount of duplication and therefore less net resource use.