I am prone to severe motion sickness while on the road, so unless the bus service offered a significant time savings from waiting for another flight I would much rather wait. Chances are the airline doesn't have a bus and driver on standby in case of a cancellation, so they would have to first find a bus along with an available driver and get it to the airport, which could take several hours.

Additionally, many bus operators, even the more reputable ones, are notorious for various safety and maintenance issues. Unless I could verify the bus operator wasn't cutting corners, I would prefer to wait. It wouldn't surprise me if some of the airlines simply went with some shady Chinatown service to get people on their way. As much as we may complain about mechanical delays with the airlines, there is nothing worse than being stuck on the side of the interstate during the middle of the night in the middle of nowhere because of some mechanical problem with the bus.