I have to agree that using coins where one could use a rewards CC is a waste. Most coins I bank, but there are some uses where cash is required or is most convienient:
1. Vending machines.
2. Metro tickets.
3. Office collections
4. Garage Sales
5. Farmers' market
6. Splitting a lunch tab.
7. Barber shop
8. Taxi
Obviously, these don't cover my $1K/month spend, but at least some coins get circulated. What the coins DON'T do is ride around in my pocket. They are too bulky and too heavy.