Well, experimental doesn't mean it isn't usable (we are talking about an experimental standard here, not about experimental software).
I run an ejabberd server and for example the Client State Indicator (XEP-0352), which is one of the extensions that improve battery life for mobile clients, was experimental for a long time but at the same time also available for major servers (e.g. community edition of ejabberd) and mobile clients like Conversations (open source too).
So in my experience, the battery consumption of a modern XMPP client is quite good. When people are complaining about the bad mobile experience they are mostly referring to the time before the mobile extensions were built.