If you have unused fare left on a metro ticket (or multiple tickets), you can go to a ticket machine and have the fare consolidated onto a single ticket, or if you have partial fare, you can add money to it. For example if your ride is going to cost $1.00 and you have two tickets with $0.50 left on them, you can either add $0.50 to one, or have the two combined into a single ticket. You can not, however, redeem them for change.
One evening as my girlfriend and I were returning to the car after a day in the city some female approached us begging for fare. I offered her our two tickets which still had a good bit of fare left on them, and most likely would have got her anywhere she needed to go.
She not only declined them, but got angry that we hadn't offered cash. It made a lasting impression on a 19 or 20 year old...
And now the government is doing their panhandling FOR them.