If no one's checking for counterfeit coins, and the users themselves don't know they exist, it sounds like a pretty awesome opportunity for the counterfeiters.
Just take a look at the number of comments here saying the same thing as you - from a counterfeiter's perspective, it sounds pretty ideal.
Unless they can fool machines, it seems very inefficient.
Launder them through your other criminal activities.
"Hey, you want to be paid for all the drugs you've sold recently? You can have 500 in 'real' or 1,000 in fake notes + coins."
Also, this is not a case of the supplier trying to con its "client", but one of choice. "Would you prefer real or or more in fake ?"
I was given a South African 5 Rand coin in my change in Amsterdam (should have been a €2 coin):
https://en.wikipedia.org/wiki/File:5-za-rand.JPG
Broke a 20 getting intoxicated in a bar/coffeeshop, then bought a kebab after on my walk back. Kebab shop was only place I got metal change that night.
I didn't notice until the next day when a coffee place threw a fuss when I used it to pay for my espresso.
I suspect they often mix them in for drunk tourists.
Adjusted only for changes in the CPI, Josh Tatum netted $121 in today's dollars every time he spent a gold-plated nickel.
(0) https://www.pcgs.com/News/The-Man-Who-Could-Stop-The-Mint
A relative who works in the industry told me he sees large numbers of counterfeit pound coins. I would guess counterfeiters might launder coins through machines. Or even, though I've never seen it, sell discount coins directly to gamblers? In any case machine recognition would be a high priority. The machines in question would be relatively unsophisticated ones with a thickness gauge and a 'ski jump' that effectively checks the weight.
One could probably go to a casino with a bag of fake coins, put 50 in each machine, and take any and all winnings. Some machines even pay out winnings in notes.
FOBT winnings are paid out via a receipt that is redeemed over the counter, so you can use them to convert coins into notes very easily. Gambling winnings are not subject to tax in the UK, so these machines are an extremely attractive money laundering method. Britain has around 9,000 betting shops, so it's easy to move around and avoid suspicion.
https://www.theguardian.com/uk-news/2013/nov/08/gambling-mac...
Note that this was a great way to get millions of coins into circulation back in the early 2000s, not so easy now.