Let us say each message exchanged in XMPP is about 200-300 bytes. If in a session (time between app foreground and background on a mobile device), user exchanges 10 messages, we are talking about net data transfer of less than 3K bytes. Each of those characters might be compared to 10 tokens while parsing (thee is scope for significant optimization). I think, the battery cost of 30K comparison on CPU isn't comparable to the cost of rendering one of those messages on the screen and it might be sub 1% of the cost of keeping the screen lit for that user session.

Optimizing parsing cost, at the expense of breaking foundational philosophy of their communication services, is hardly justifiable. XMPP has several overheads, XML will not top that list.