On the BART back from downtown San Francisco yesterday, only 2 of the 60 people in my car got off at Orinda. Only 3 of the 60 people in my car got off at my stop--Lafayette.
The reason is clear as you get off at the Lafayette stop in the median strip of the ten-lane California Scenic Highway 24: the big electronic sign says:
OAK ARPT: 19 MIN
DWNTWN SF: 22 MIN
SF ARPT: 27 MIN
$10 dollars, 40 minutes each way, and you are very likely to have to stand for at least one of the legs as opposed to 22 minutes each way and then having to brave the parking situation downtown.
It simply doesn't make individual sense unless your trips to DWNTWN SF have to be made at the peak commute hours when Teh Bridge is "backed up"...