You can still simply go sub to the large communities even if you're not on that server though.
I'm curious to see how that whole hierarchy pans out, how will servers organize, will it end with basically if you have a large community you have a dedicated server for it, but maybe people actually prefer to register with a server that they identify with more from a locality or mentality perspective, rather than topics?
We can speculate but ultimately it'll be a natural evolutive process. I know for myself I picked something that I felt would be general purpose and I suspect a lot of users would feel similarly.