Whatever it is, I'd consider it a bug that needs to be fixed. I know it's been said before, but something like Tasker can help with that. Add a rule that whenever you arrive or leave a certain location, start or stop an app. If you're carrying on long conversations on a PC and your phone is constantly receiving the messages notifications and reacting to them, yeah I could see it consuming some juice. But AFAICT that's not how it works on my Rez, where after my Rez is woken up from sleeping there's a flurry of activity as GTalk displays all of the messages that came in since it was last awake.