I've always wondered whether it was an intentional feature of the BART machines that they focus on the cash value on a card rather than the trip cost. Underpaying isn't allowed--the gates at your destination won't let you leave--and overpaying puts petty change into the BART's coffers when you forget about/lose/toss the card.
Not to mention the need to add 5c to your ticket if you want it back at the end of the journey - say for expense or tax deduction purposes if you are making occasional but expensive long-distance trips for work assignments. I've spent >$300 on BART fares for a month-long project before, and if you exit the station with the exact fare on your ticket, there goes your receipt.
For plastic yes, you're right. I tend to use cash for transit fares, I don't know why. I must have some mental block about it, because I'll even leave the station to go use the ATM, then come back and make change and then put the $5 bill in and grumble about having a pocket full of quarters. Oh well, I fail.
One time I got stuck inside a BART station because the machines on the outside take plastic, but the ones on the inside are cash-only. I was 40¢ short and had no cash. The person at the station let me through though.
Another one of their design decisions that makes no sense :)
