The point of the whole free argument isn't making it easier to ride, it is to remove the overhead of having to gate access. If the bus just let people on and off and never had to do an exchange over riding it they would be more efficient.

A nominal fee like a quarter might be a good idea. I lived in Santa Monica when their buses were 25 cents and don't recall many issues with homeless or other people abusing it.

It depends on the location. In my neighborhood in SF there was a guy who dressed up like a giant viking warlock with a leather pouch of coins that he would hand out to any homeless person who needed fare for the N-Judah.