This is a repeat from The Urbanophile, but he makes a great point that should be repeated:

Why have a fare in the first place? It is odd that we pay per use on transit. We don’t pay to check books out of a library. We don’t pay to visit most city parks. We don’t pay when the police or fire department come to our house for a legitimate emergency. Most non-utility municipal services are provided for free to users and funded by taxes. So why is transit different?