Of course, it should go without saying that nobody should walk away from the pump for much longer than it takes to fill the tank, if only because it's a douchy move to tie up a gas pump for longer than you need it. But if shutoff failures were anywhere near as common as you seem to think, the latches would be prohibited everywhere.
Let's see. It looks like about a million people a year die of lung cancer, with about 90% of those attributed to smoking. But we still haven't seen any statistics on the number of burn victims and damaged/destroyed gas stations due to popping into the 7-11 for a Slurpee while the tank fills.