I see this as a special case. For most disasters there is no problem with clergy being in attendance as long as there is no risk of contaminating a crime scene, but a bombing or explosive fire is an “ongoing risk”, in which at any moment things can get much, much worse.
Remember that video last week of the people in Texas filming the fertilizer plant burning? They were 300 yards away when the plant exploded in earnest, (at about 1:20 on the video):
http://www.youtube.com/watch?v=jzDC3iKbTzY
In the case of terrorist bombings, it is common practice for bombers to set off a small bomb, that causes people to flee to a place away from the blast and cluster together, where a second bomb is concealed, larger, and intended to make far more casualties.
They also make bombs that target emergency responders, such as firemen and police, figuring that if they take them out, it will kill many wounded people who cannot get emergency first aid.
So in the case of a bombing, emergency responders try to give essential care on scene, like stopping arterial bleeding that can kill in seconds, but then immediately transport wounded away in different directions.
This exact thing was seen at the marathon bombing, as a man missing both his lower legs was being carried off, another man pinching his exposed artery shut while carrying him.
Ambulances are told to park away from each other, because terrorists will often make an ambulance truck bomb.
How can priests fit into this situation? Only in the way they can do so in the middle of a battle in war.
They do so willingly. Priests ministering to those in the middle of a chaotic and dangerous situation know the risks. Fr. Judge knew the risks, but he also knew he had a DUTY to be there, and be of comfort to those who may not make it out.