I think that airports do need buses. After all, people's journeys start and finish at homes/offices/hotels, not airports. The best option is to have a bus interchange next to the airport for all the local buses; people then realise how easy it is to get to the airport by bus from the local towns, and will be less likely to use rival airports further away that they'd previously thought were easier to get to. It's important also to remember visiting inbound passengers, who won't be collected by friends or family, but will rely on public transport to get them to the town/city.