You must have never done anything like -say- use the 38 to connect to CalTrain shortly before or shortly after rush hour. :)

I made the 38->30/45->CalTrain or 47->CalTrain ride every single day for nearly a year. It seemed like once a month or once every other month, MTA fare inspectors would board the 38, 47, or -far more rarely- 30/45 I was riding and check everyone's proof of payment. On the 38 and the 47, it always took about five minutes and was a... fairly irritating delay on top of a really irritating commute.

I mostly take the 7 (nee 71), 6, and 21.

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.

The Fare Inspectors will fine you if you haven't bothered to swipe your card, even if you have a monthly pass. The card stores a record of the last $WAY_TOO_MANY interactions with a Clipper Card reader, so it's trivial for them to check. Those guys have no sense of humor. :(

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.

I took the 27 to work occasionally, and because it stops right outside the police station, fare enforcement occurred like 30% of the time