An interesting thing is the Push Notification system used by Firefox OS which does not maintain a connection with the server like Android or iOS, instead the carrier has a notification server so that when you send a push notification, the carrier sends a signal to the phone and awakes the app.
Making IM apps that don't need to keep a connection open is a great way to reduce battery. This way whenever you receive any message the carrier pings the phone, the app awakes and open the connection to retrieve the messages.
I am sure that this can be used to make a XMPP client that makes good use of the battery by only keeping the connection open while people are talking and effectively closing it once the chat goes idle.
As I wrote, it is hard to keep the conversation private in such an offline+push context. On the other hand, the additional battery load induced by a properly implemented XMPP connection is low. Apple's main fear is probably badly designed apps which really drain the battery.
Making IM apps that don't need to keep a connection open is a great way to reduce battery. This way whenever you receive any message the carrier pings the phone, the app awakes and open the connection to retrieve the messages.
More info at https://developer.mozilla.org/en-US/docs/WebAPI/Simple_Push
I am sure that this can be used to make a XMPP client that makes good use of the battery by only keeping the connection open while people are talking and effectively closing it once the chat goes idle.