I'm never sure if the people who attempt to use this reasoning are just trolling or if they can't/refuse to see the big picture.

If everyone jumps the fare, metro has a revenue shortfall which could result in among other things: service cutbacks, fewer employees to perform maintenance, safety checks, etc...

So yeah, it harms all of us when some people decide they don't need to pay for a service that depends on our revenue to function properly.