Any coin is worth imitating if can be made for cheaper. In ancient times this usually means if it
had a significant fiduciary value to it.
Even though a
pentanummium might not be worth much itself, if you can make a bunch for less than they are worth you can make profit.
The most commonly counterfeited UK
denomination is apparently the 1 pound coin.
The size reductions of the early
Byzantine coins could have provided lots of opportunity for such mischief.
SC