Absolute nonsense.
If I can exchange 50 copper denar for 11 gold denar, then you'd have to be pretty daft to spend the 50 copper denar on the bread. If you expect the exchange rate to be 50 copper denar for 11 gold denar in the near future, then you'd have to be pretty daft to spend the 50 copper denar on the bread.
If the exchange rate is currently 50 copper denar for 11 gold denar and you strongly believe that it will continue to be 50 copper denar for 10 gold denar for the foreseeable future, then it doesn't really matter at all which one you spend.
If people truly desire the gold denar more than the copper, then the exchange rate (and the price of merchandise) will reflect this. The merchants will price their merchandise cheaper when spending gold denar so that they can acquire what they actually want (gold denar) without having to pay exchange fees. The customer will benefit by spending the gold denar (since he'll get a price cheaper than spending copper), and the merchant will benefit (since he'll avoid paying exchange fees to acquire what he desires).
Of course we're talking in metaphors here, because the Romans didn't have the benefit of electronic exchange and a global marketplace for use and exchange of their currency. As such there was a LOT more friction associated with such transactions than there is with bitcoin and federal reserve notes.
What you've done here is gotten it completely backwards. If 50 copper is worth 10 gold today, and 50 copper is worth 11 gold tomorrow, what just happened? THE PRICE OF GOLD WENT DOWN, AND THE VALUE OF COPPER WENT UP.
You have completely reversed the conclusion by completely reversing the scenario. We were assuming that gold was "good" money and therefore going up in value, and copper was "bad" money and therefore going down in value. Now you have come in and said, "ABSOLUTE NONSENSE! IF COPPER IS GOING UP IN VALUE AND GOLD IS GOING DOWN IN VALUE, OBVIOUSLY I'D SPEND MY COPPER ON GOLD FIRST!"
Sure… i guess… maybe… but that is not what I was saying at all. If gold is "good" and copper is "bad", then I should expect that gold will be more valuable tomorrow than it is today, and I should expect that copper is less valuable then it is today. So if 50 copper is worth 10 gold today, then tomorrow 50 copper will only be worth 8 gold. See how you got that backwards?
Because you get paid on the first of the month, and you won't be buying the bread until the 20th of the month. You don't want to hold WORTHLESS VALUE LOSING DEPRECIATING IN VALUE FEDERAL RESERVE NOTES for those 20 days, do you?
You'll be buying gasoline for your vehicle on the 29th of the month.
You'll be paying for your car insurance 3 months from now.
You'll be buying a sofa in 8 months.
Sure, if there are things that you KNOW that you'll spend money on in the next few days, you'll keep that as federal reserve notes, but all the rest of it you'll convert to bitcoin immediately upon receiving it. Since your employer will be encouraging their customers to pay with bitcoin (by offering better prices for the more desired currency), you'll negotiate to have most of your salary paid in bitcoins (so that both you and the employer can avoid the exchange fees).
Here is the main point of Grisham's Law: You want to spend your debased currency first and fastest. You want to hang on to your appreciating currency and not spend it, you want to hang on to it as it gets more and more valuable.
What you are suggesting is that all debased currency should immediately be exchanged for "good" money and then only the "good" money will be used. Yes that's one strategy, and would probably work if the government weren't' forcing the debased currency into use. Im just saying Bitcoin is made to hold value by design, if another currency that is made to hold stability and therefore become a Unit of Account by design, it would make sense that people would have their savings in Bitcoin, and their daily cash in the Unit of Account coin.
Different properties serve different functions and meet different needs. Bitcoin does not have to do everything.