The cases I've read about were of the form "app A asks app B to do something it can't via the Intent system". That sounds scary until you realize that a standard example of this is an app that can't access the network sharing something via email. In other words, app A has transferred control to app B and what the user does (or doesn't) decide to do with app B is their choice, not app A's.
Indeed. Delegation via intents makes things more secure as broken code can be patched in one place rather than in many. And, you get tighter control over what apps can do: if you never want an app to share something via Facebook, simply uninstall the app that provides the "share via Facebook" intent.
Interesting point. Sometimes I find myself wanting to keep the app, but drop the intent. Usually that is to shorten a list, but not always. I'd love to see low-level intent-blocking (as well as low-level, fine-grained permissions blocking, but that's a whole other story).