The examples given, from what I can see, are about accessing private feeds. Feeds can be public. How would someone access someone else's feed? One way would be with direct REST or Json, but how with the API? Is there a way to do so with my own username connection? Can public feeds be accessed without the key for that feed?
One use case would be a game in which scores are shared by different users. Another might be two persons with weather stations or other sensors who, for reasons of research wish to share by means of public feeds. This introduces another more complex to implement feature which would be shared feeds where a feed is not public but it's owner deliberately shares it with particular users.
A fun example might be post notifications from this forum. How many posts? Last Post?