Since any system that uses digits needs a minimum unit containing a terminal 1, in order to remain accurate, the penny serves a vital purpose. It allows fine adjustment of value that counting by 5s doesn’t.
Consider this. Every sale of gas that is pumped in the US shorts the customer 1 to 5/10th of a penny, half the time and gives 1 to 4/10th back half the time. Every 10 sales, they make an extra cent. Now multiply that by the number of gas sales per year. Now imagine if you rounded every purchase price in America to the nearest nickle, biasing up. Or every sales tax percent that rounds up. It may not seem much, but across the whole population and economy, those mils and two cents add up.
One of the most effective embezzlement schemes I ever heard of was a guy who programmed computer accounting software for a bank. He set it up so that any time a transaction or interest payment was rounded unevenly, it put the spare 1/2 cent in a dummy account at the end of the list. He was only caught when some guy named something like Zzyxz opened an account and reported its unexplained growth.
That was a plot device in at least two movies: The excellent Office Space and the execrable Superman III. I don't know if those screenwriters were inspired by a real scheme, or they were just that creative.
How finely do you want that mill to grind? For a large chunk of US history, the smallest unit of coin didn't have a terminal 1. It was a half-penny, with a terminal .5. Should we resume minting half-pennies? Mils? Nanocents? Where does the madness stop?