On competition I do as I know my horse can be more stressed and unpredictable because of the environment. But at home I don't.
I plan to buy one but it is really (really) expensive... But I know it can save a life so it worths its price!
When I start riding a horse I really don't know, I wear one. I feel more confident with it. On competition I have a my airbag vest or a back protector.