My read was that the SIP protocol was just less well suited to a mobile environment. Not merely that long-lived connections are bad, but that it would be inappropriate to use a long-lived connection that requires keepalive packets or loses user data every time the user goes underground (SMS queues up for delivery when the connection is re-established, and APNS will at least deliver the most recent push). Technically even the SMS solution requires a persistent connection, it's just the device's persistent connection to the cell network, which is presumably better tailored for the mobile use case.