I have read instances in history where gold proved useful. These tend to occur during wartimes, when the currency system has collapsed. You need to get on the last boat/train out of town before the hostile army arrives and war breaks out? You just might need to bribe someone, since inflation has made paper money worthless, you need to offer something else. Under such circumstances, you won't have time to weigh it and check purity, etc. It's simple: you just have to convince the person in position to help you that you're giving him everything you've got.

You'd probably know if you're going to be in such a situation. Fortunately, most of us have no use of gold in our PSK.