My little guy gets spooked really easily, that harness has kept him from taking off more than once. He is also a very large & strong child, if he decided to take off, I doubt I could hold him back. he turned 3 in march, he wears size 6 clothes.
We don't really use it anymore, he listens better now. but they are not just to keep him from taking off, they prevent kids from being taken as well.
My parents use a harness on me, and I am a very well adjusted adult. The important thing is to use the ones that go around the chest. I remember seeing the wrist band ones years ago and thinking that's a good way to break an arm.