So in specific terms, it is an extension. f:event is sufficient if the purpose is to perform processing at the start of the request. s:viewAction (and eventually f:viewAction) is designed for when you have to perform logic to verify that the view can even be rendered. View-level security is one example. Another is verifying that preconditions are met. And the key is to make navigation away from the view an integrated part when it's determined that the view cannot and should not be rendered.

-Dan