Part of the idea behind "more accurate contamination maps" is to mobilize the appropriate response -- personnel, drugs, and so on. So it isn't just about knowing where to clean up (although it involves that, too).

These sorts of tests also aim at helping guide the first response. For a long time, there was an ongoing debate about the proper response to an attack on a subway (it's probably still going on, but I haven't talked with this crowd in a few years). For example, suppose a CCTV spots a bunch of passengers going down in a particular terminal. Should you shut down the ventilation system, in order to try to contain the material? Or should you crank up the system, in order to dilute the hazardous material? How should those decisions change depending on the type of material, the amount released, the atmospheric conditions, and the timing of the response? Should you stop all trains in the tunnels, or should you run them to the nearest station for evacuation? All of this requires estimating potential health impacts, to those on the platform, in the rest of the system, and downwind of the affected stations.