Time and again the proposers of these systems forget about how easy and vulnerable they are to hostile action. Military systems are targets in times of conflict and expecting the enemy to take pity on one of these lumbering hybrids is not a realistic hope.
Actually, the opposite is true. Both the US and UK militaries have fired rounds of varying calibres - up to and including 20 mm - into HAV-made vehicles and have even tested the effect that a SAM might have, in a bid to determine their vulnerability. In each instance, the airships survived, in most cases so well that, faced with similar ammunition in combat, the crew probably would not notice that they had been the subject of an attack.
There are several reasons for this, chief among them the fact that helium is an inert gas and so incapable of igniting even when exposed to a tracer round or missile detonation. While the hull's fabric was pierced by both the entry and exit passage of the round, the gas inside was contained under such a low pressure (1/10th of 1 lb per square inch) - and there was so much of it - that although the hull was riddled with holes it took hours to deflate significantly.
It was concluded that a SAM would pass straight through without detonating, leaving two relatively small holes. Tests established that it would take the vehicle three and a half hours to deflate with these two holes in the skin.
A warhead was also strapped to the inside of a fully inflated test hull and detonated to test the airship's behaviour if a SAM were to explode inside the envelope. The results of the experiment were somewhat surprising, blowing out the windscreens out of the testers' cars, but having the hull just going 'boing' and coming back out again. And although the casing from the explosive made a number of shrapnel holes, they were irrelevant.
Even if such holes were numerous, they would have little effect on the vehicle as the helium would not be escaping under pressure. As a result, the damaged fabric skin has a natural tendency to seal itself. Also, as there is no internal structure to the envelope (it is fully supported by the pressure of the gas), there are no parts to damage.