I think you're essentially on the mark with your assumptions / questions. The one's that are typically sleepy are the battery powered devices, like @thebearmay indicated. But as you say, if they have something to report, like temperature swings, then yes, those that report temperature or are triggered often, will for that reason report back more often.
In terms of battery life and alike, I have Smartthings multi-purpose sensors (now produced by Aeotec) on most doors and windows and, depending on different factors can get anywhere from 6-18 months of battery life for a CR2450 coin battery. So it does become expensive the more of these you add. The condition of your mesh can also play a part in the battery drain, if the sensors are having trouble communicating back to base, it can significantly drain the battery. So for those low-use areas or those that do not have good coverage for the protocol you choose, you may want to look at mains powered options, not something I am familiar with myself.
