As I posted earlier the POH does not set out a limit but it does say that an average pilot can operate safely upto 15 Knots. This is not limiting but a recommendation. My groups policy is however limiting and I will respect the owners wishes when operating their aircraft.
Since I only have 1 hour PIC on type I don't consider myself above average!
As to this being an arbitrary number; aviation is full of them, whether it is in the definition of VMC/ IMC or licensing hour requirements etc. I agree that these are arbitrary numbers but they have been set out for our benefit based presumably on others past experience or hind sight.