There is always Caltrain. Anyways, even if Bart went all the way to south bay, I'm sure these buses would still run as Bart would have too many stops, would be too slow and uncomfortable. If you have a lot of people coming from the same places and going to the same place, why not provide a dedicated bus that is virtually point to point?

Seattle had (has?) express buses to Boeing that worked under the same principle. These were provided by the public transit agencies, however. Then Microsoft came out with the Connector, which is similar to the Google model.