Indeed, initializing a session is a heavyweight operation. But something polling every 15 seconds is likely never going to tear down the session. From what I've heard the timeouts tend to be a bit longer than that (e.g. 30s), exactly for that reason.
And yes, there could be physical limits as well causing an individual cell to be fully congested due to open but idle sessions. Either way the polling is causing problems disproportionate to the amount of actual traffic.
