The time and energy that will be used to process said coins will excess their face value.
There are people with nothing better to do who enjoy sorting old coins.
Personally I prefer untangling pieces of string or yarn. I don't normally have time for it, but on the rare occasions that I've had nothing "better" to do I've really enjoyed it.
If that were true then the charities wouldn't take them in the first place. They're (ahem) not doing it for charity.