Some promises have value, while others do not.  Which are which will vary over time.  The same is true of other commodities, although promises are typically more volatile.  Gold certificates have been successfully used as a long-term store of value and as an exchange medium for high-volume trade -- for suitable values of "long-term" and "high-volume".

Physical metals have persisted in storing value longer than anything else, and were used for essentially all trading worldwide for centuries.  Arguably, some of the features of gold certificates which make them more usable for larger volumes of trade (in excess of say, a million dollars in value, pick a number, corresponding to the strength of your back) also contribute to their inferiority as a value store, relative to physical metal.  That does not mean that they are not suitable for both uses, within their actual limitations.  Gold or silver certificates which are fully backed by deposits and have the full faith and credit of their issuer are a pretty darn good value store.