Just to be clear there are only two types of actual US currency out there. The paper stuff and a leder of what 'banks' (or governments etc) control maintained by the FED. So, now you have a seperate DB not actual cash or physical currency. Which means you have counter party risk. Now if the US government can decide to insure 3rd party's which is almost as good as cash but as anyone that has delt with a banking error knows there is a world of difference between cash and IOU's.