Your criticism is valid, and means that salt isn't viable in the long run-- but you do understand that the word 'salary' is derived from the fact that Roman soldiers were, in fact, at times paid in salt? Technically, they were supposed to get money to buy salt (the reasons for this practice remain debated), but at times, this was just cut out altogether, and they were just paid in salt directly. Not to mention that due to the situation of 'salt money', the soldiers apparently used salt as an informal currency amongst each other, even when "real money" was available. So salt, too, was historically used and does serve as a real example.