We could offer free ridership but still use orca cards and ban people who misbehave or befoul the place. Whether we keep problem children off appears to be wholly orthogonal.
Over here in my European town this isn't an issue as we have a "trust based system" where tickets are only checked infrequently by spot inspections on the running bus and most people have a monthly pass. So it's just hop on and off.
In my experience, being able to pay at any of the doors increases throughput because people are not bunched up in a single door, neither to get on or off, Parallelizing (and load balancing!) the movement of people. Not having to tap to pay (either because it is free, because monthly passes don't require it or because you can pre-pay at a machine or online) would have some additional time gains at rush hour on popular lines.