public transit is actually a fairly small percentage of the way people get to and from airports. SF is one of the highest in the country though, I think around 22% of people use transit to get to and from the airport. However, on the opposite side is Cleveland with 5%.

Regardless, there are a lot of people who may be going to a suburb and not a hotel by the BART stop that don't want to do 3 transfers and would rather take a shuttle. Until a multimodal search engine can show me those options, I'm not really interested.

As a bay area native, I know no one who has taken BART to SFO. It is either drive oneself, get dropped off, or some form of shuttle.