Come to think of it, the best use of dogs is almost certainly for quickly sniffing at lots of things, e.g. all the bags just as they are going onto the belt or passengers walking through the green channel.

If they carefully swab down your suitcase and then test the swab in the spectrometer and it comes out negative, a dog doesn't stand much chance. Dogs are not magic and they can't sniff through things: if a 1kg brick of coke is carefully wrapped in your suitcase in such a way there is absolutely no trace on the outside, a dog has no more chance of detecting it (with the suitcase closed) than I have of magically seeing through a brick wall.

Of course, if you have some small traces of something dodgy on your clothing, they can't swab down all of that and a dog might well smell it. But for testing that sort of thing on suspicious individuals is exactly what they have those puffer machines for. As I say, the advantage of dogs is they can quickly give a quick sniff at everyone, whereas swabbing down or puffing of passengers takes times and can only be done properly with a few (either randomly selected, or especially suspicious) individuals.