If you look in car related forums, you'll find that valet joyriding stories are not that uncommon. Every few years, a car dealer valet crashes an expensive car while joyriding it and the story gets cross posted to every car related forum around.

And then I remember someone who worked as a valet telling the story of them trying to learn how to use a manual transmission on a customer's car because none of the valets knew how to use one.

Then there is the added risk of theft. The only time I've had things stolen from my car was when it was valet parked.

I consider valet parking to be like checking baggage for an airplane flight -- something that may be necessary in some situations (not enough parking, so valet parking is needed to allow "burying" of cars), but generally to be avoided.