I agree with the fact that doing it at Defcon is ten times better than doing it at, say, LeWeb.
But I question the overall value of this activity for the society, compared to other things.
I guess not everyone is as altruistic as you, dedicating their time to societally useful things like talking about intercepting cell phone communications on HN.
Some of my friends bring burner/throwaway/blank phones to Defcon, and this is why.