So if I understand this right, you pick a server and your server account can post on that server and any servers that that server federates with.
So what happens to your account if the server you joined goes down? Yes, you could always create a new account somewhere else, but you lose all your followed communities and post and comment histories as well.
This is true, which is why a solution will pop up maybe in the form of a website that could track what servers federate with what. People could choose a completely open server or a server that defederates the volatile ones.