People bring this up as an objection all the time, and I think it always comes from people who have never actually had a car with this feature. First, as mentioned earlier, every car I've had with pushbutton start will not let you start the car unless the key is INSIDE the car. Second, if you try to drive off with the key NOT in the car, they've all made an alert sound and shown a message on the display that the key isn't in the car. It's certainly possible to intentionally drive off without the key, but I don't think it's possible to do so accidentally (or if you're so inattentive that it is possible, you probably shouldn't be driving in the first place).