Currency has value because people store it. That is, it has value because people do not immediately liquidate units of A to get units of X. If no one wants to hold A, A has no value. People hold currency/gold/shells because they anticipate that it will have value in the future. Collectively, this gives something a global value. So, I reject the notion that value comes solely from ease of convertibility. Gold is a perfect example.
I disagree. Something has value because people will buy it. A currency has value because people will buy it in exchange for goods and services, or other currencies. People are willing to buy gold, thats is the reason it has value.