What special event would terrorists want to disrupt that would require their cars to sit around the airport for a few days in a parking lot? This would only deal with the home-grown (domestic) type. We only have had one major incident of that, and it focused on a government building. Yeah, I know an airport is owned by the government, but the difference is clear.
These guys are called suicide/homicide bombers for a reason. They could drive five minivans in a row that are laden down with explosives right up to the terminal, across the curbs, through the glass, into the counters, kapow, and that would be it.
They are evil and crazy. Do not ruin our lives with ineffective security. Look how much money they are spending/waisting just to put out "window dressing" security. It is largely a jobs program.
I will concede that there is middle ground. Perhaps they could walk bomb sniffing dogs around the parking decks. Otherwise, use all the parking decks.