Yes, in airplane mode it uses much less battery. But if you're not getting a signal anyway, you won't get texts or phone calls so might as well put it in airplane mode while you have no signal.
An example-My Razr Maxx is not activated (because I'm using my Razr M instead). But I still play with my Maxx on WiFi. If I don't put it in airplane mode, then it kills the battery because it is desperately searching for a signal that it cannot connect to. That phone on airplane mode with WiFi on lasts four days. Not an airplane mode, it only lasts hours.