Not so agreeing on the Diplomats, since they don't require hundreds, let alone thousands, of gold on person unless you're doing a Hireshmont-style tour of the continent. At least in my experience. Doubt expenses have spiked while I've been gone.
Spending 10,000 gold buttering up foreign dignitaries, making friends, supporting philanthropic causes, and generally being a goodwill ambassador is very, very easy to do. I can testify to this, having done it more than once.
Priests SHOULD be able to be ADVANTAGED gold-couriers. That works well for game balance, it works well for making the class more fun for players, it is historically accurate, and it has no evident downsides that I can think of. There's no threat to game balance in allowing a priest to carry a large amount of gold unthreatened. There's no improvement to anyone's gameplay experience reducing the utility of the priest class.
The burden of proof should not be on those of us who think we should be able to carry gold freely; the burden of proof should be on those who believe there "should" be a random chance of bandits/pirates/robbers/whatever. In what way does that improve the game? For what reason would we have that additional feature?