Since the 1980s, the vast majority of vehicles have anti-siphon baffles that prevent a hose from extending down into the tank.
However, here's a scenario where you might want a locking gas cap, even if your car has the anti-siphon baffle that renders it unnecessary.
During the gas crisis in the late '70s, I found myself in a blocks-long line waiting to fill up at a corner station. The station was on the South-West corner of the intersection and we were all lined up down the street to the West of the station.
Everyone was being polite (it was early in the first day of the shortage), waiting their turn. The next-to-go car would wait at the street, ready to drive into the station when a pump was available. I'd been in the line for over an hour and was about three cars from the driveway. Suddenly, as a car vacated the pump, and the next driver started to pull into the drive, a jerk in a Cadillac -- traveling South on the street approaching the station -- cut diagonally across four lanes of the West side street, directly in front of us, and pulled into the station and up to the pump, ahead of all those waiting in line.
As he hopped from his Caddy, hoots and hollers and a stream of profanity greeted him. He ignored it all, stepped to the rear of his car, removed his gas cap and reached for the pump. With a visible look of disgust, he realized he had to go into the station to pay for the gas. And this he did, despite the ever-growing outrage behind him.
For a moment, we in the line looked incredulously at each other, trying to decide if the situation warranted (or would justify) a face-to-face and possibly violent confrontation. But as the murmurs for blood grew louder, the guy in the front of the line decided to act.
He opened the door to his gas tank, removed his locking cap, strode purposefully across the lot and slapped it onto the open line of the Cadillac. To tumultuous cheers, he returned to his car, got back in his seat and waited the return of the Caddy driver.
It took the Caddy driver a few moments to realize the hopelessness of his situation and a few more to get his money back from the attendant, but he eventually drove away with cheers and jeers and raucous laughter ringing in his ears.
And that, I'd say, was worth the investment in a locking gas cap.
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