While the use of XML in the protocol might be considered verbose, stream-level compression from TLS quite makes up for that. With many contacts, the number of presence stanzas might indeed drain the battery because the antenna will be 'up' quite a bit.

However, the XMPP community has been active in quantifying such issues and providing solutions. The beginning of a document with background information is available as http://xmpp.org/extensions/xep-0286.html. There are also various opinions on the topic to be found online, like http://www.deepdarc.com/2008/02/14/mobile-xmpp/, that, as you can see, dates back half a decade already.