I've been through a few airports since 9/11 and I've yet to see dogs patrolling security checkpoints.

You can't use dogs for static defense because they can only stay on task for so long. After 10-15 minutes of bomb sniffing the dog needs to take a break. You would need several dogs (and handlers) per checkpoint, which is cost ineffective.

If we had systems that sniffed for chemical residue of explosive compounds we wouldn't be having an argument about ineffective and silly porno-scanners, because they wouldn't be used.