Just as a discussion point - Federation doesn't appear to add any capability to GeoEvent Server unlike other server licensing roles. Federating GeoEvent only serves to integrate its security model to Portal. You still have to create Enterprise/Server connection(s) within GeoEvent Manager regardless of whether you federate or not.
As far as I can tell, there's no loss of functionality from not federating your GeoEvent Server. Even though it is not a common setup, there doesn't appear to be a significant drawback.
Timo