Earlier quoted context omitted.
Anecdotally, at least, I see a lot of people riding Muni who are fare evading. I've also never seen any fare enforcement on a Muni bus, despite living here for ~6 years.
I have a monthly pass and can't always be bothered to swipe my ClipperCard. It would look to you like I'm fare evading, but I'm not really. Perhaps others are doing the same.
I'm a huge fan of Caltrain's policy: Monthly pass holders only have to interact with a reader twice at the beginning of the month [0]; once when you board and once when you disembark. For the rest of the month the conductors use their brains to remember the current month, and compare that against the end date on your monthly pass.
[0] And -realistically- you don't even have to do that. I went a couple of months without interacting with the Caltrain readers; the conductors never said a thing.