Would such an app actually help? Is there, for example, a difference in power usage between using a 3G connection for data and merely keeping it open and listen

Would such an app actually help? Is there, for example, a difference in power usage between using a 3G connection for data and merely keeping it open and listening for calls? If there is, those 30 minutes between checks for Wifi could potentially be very taxing for the battery. Real world tests would certainly be interesting. (I only have an old dumb phone so I have no idea. I do see substantial differences between using 3G on my iPad and using Wifi — 7h vs 10h — but when I'm using Wifi I turn 3G off since I'm on a prepaid plan.)