I've wondered about this for a while.
What is the point of an indestructible object? I'll admit that it adds an interesting twist to the game as you'd better know which of those buildings in the town are indestructible else you'll use up all your ammo needlessly. I've still never understood the philosophy of designing in these indestructible objects. Also, if a GV pulls into one of them, are they protected in there? It seems I've seen them killed but I can't be sure. Either way, it seems a bit gamey to me to have these IOs around.
Just curious
-hb