Almost never. You've got the Gardens exception in your question. That's the most common exception. I could construct a couple other contrived examples where it might be good (though there's still probably something better to do), but the real answer is never. The only good reason to have so many copper cards I can think of is to keep them getting too worn too fast if you don't use card sleeves.
Can't believe I forgot Goons! Especially if you have multiple Goons in play, near the end of the game you should buy anything you can, including as many coppers as possible if you're out of money but still have buys. Similarly, if you're running a lot of Bishops, you can use copper as fodder for them (though here anything with value >= 2 is preferable). This (speaking of statistics) probably explains this graph. If a player is buying >10 coppers with Goons or Gardens in play, it's no surprise they're winning.
I once played a guy who opened Chapel / Silver, I opened Swindler / Silver. In the next few turns he Chapel-ed 5 coppers and bought a Village, I swindled his Silver. He had to buy a copper to afford another silver.
Sometimes, if you're drawing most of your deck and you have an old Ambassador in it, it can be fun (and obnoxious) to buy a curse to use the Ambassador on. If the curse pile is empty, copper would be the next choice for this.
You could try some strategy relying on a multiple copies of Coppersmith, Money Lender, Counting House, or even Philosopher's Stone or Bank (though in the last 2 Silver would be strictly superior). It would be hard for any of these to be competitive, but you can try.