Agreed. Whether it's "I don't have service" or "My battery is dead," there are a number of conditions that keep a recipient from seeing a message right away, all of which are (usually) temporary.
Relying on "delivered" takes care of all of those situations, and is much more elegant in the UI than adding an "auto responder" message.
Does 'Delivered' show up when the message is received by another iOS device? e.g. I often leave my iPad at home but still connected to iMessage, and if that receives the messages my iPhone doesn't, wouldn't this give a false impression of delivery to the sender?
iMessage's are sent to all of your devices. I have iMessage on my mac, iPhone & iPad. All 3 devices receive messages sent to me. That's true even if one of the device is turned off for some period of time. Apple, did seem to go through some growing pains where this didn't always work as above.
To answer your other question the delivered flag is set if any of your devices receives the message.
Relying on "delivered" takes care of all of those situations, and is much more elegant in the UI than adding an "auto responder" message.