Of course, you're right: it is absolutely feasible that such a valuable trip is possible and happens. You're right that a truly valuable trip is rightfully charged a fare. And finally you're also right that such trips are probably unusual. I would go so far as to say rare.

What's much more common than that rare scenario is that, after paying a fare, you get down to the platform only to find that there's a long delay in the system, there are too many people trying to get on the trains, or something similar that causes you to leave the station to find saner transport. I've been bitten by this on BART, more than once. At stations in the heart of the city you "start the trip" triggering the fare long before you can see any of the platforms to know whether or not you should enter.

Back to the point. The problem isn't that you're wrong in asserting a valuable excursion trip is possible, but that you use that fact to rationalize the charging of a fare while ignoring the much more common cases where you'd charge a fare while delivering no value for money. Once you consider the complete picture it becomes clear that the right thing to do is build policy around the common case, not the rare/hypothetical one. It would be better to eat the cost of the rare valuable trip BART should charge for so that they don't charge people for trips they couldn't actually deliver.