Except that, no matter what, the smallest value is still one coin. So, really, all you are saying is that the coin would be exactly the same with different colors. See, if an apple costs one and a magic helmet costs 20,000 apples then the ratio will be the same regardless of what intervening coins the items are priced in. In a one currency system all you have to do is look and see, "Ok that helmet is 20,000 and the apple is one" whereas under your system it would be more like "Ok, the apple is one and the helmet is worth two of a more valuable coin". The value ratio is the same, but much less obvious. Imagine if stores in real life started pricing things that cost 100+ euros or dollars or whatever as costing, for example, 4.5 "big dollars" instead of 450 dollars. What would be the point, other than to confuse people.
They can change that in two ways. One having a barter system, two in having local scrips with gold as a leveling universal factor. Most transactions at a low level would be handled by the local currency, with gold as a more valuable currency that applies everywhere. The bank can change your money to local currency for a small fee. Other than that, every quest should have the option of being paid in gold, or the local currency, with you getting more of the local currency because they know you can only spend it in their town.