Actually it was not, back in the imperial ages of ancient times, they were only allowed to use the special minted coins issued by the kings, and no other forms of coins.
I heard stories that people were forbidden for example to use the enemy county's gold coins, even though they were both gold, just for political reasons.
You obviously forget that you can always melt gold coins into bullion. Don't know about "the imperial ages of ancient times" (Ancient Rome?), but in medieval Europe you would go to the royal mint and they would mint legal coins from your bullion for a small fee...