> I don't know how accurate Android's power estimates are, but I consistently have people telling me that they are switching back to proprietary messengers because every XMPP client they've tried drained their battery. Whatever causes it, there's a real problem here.

You have to differentiate between the specification and the implemetation. Nothing in the XMPP specification prevents you from implementing a battery friendly XMPP client. But some implementations suck/have room for improvement.