Right. But when I have suggested a number of different XMPP clients to people, and every single one of them has caused this issue, then there's a problem.
Where that problem is, I don't know. I'm just going by what I hear from people more experienced with the effect of XMPP on battery life.
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.