I have witnessed dust storms in the deserts of Iraq that have lasted for three days and even times when it rained mud! These are the extremes of war that require the ability to have options with your optics and in field maintenance sessions to manage the serviceability of weapons and equipment.
I have had to use a substandard sighting system during counter terrorist diving and water operations missions that could effect performance because better optics were not waterproof to the required standards.
Because of this diverse area of operations and the nature of special operations, sighting systems for military application need to be scalable and redundant in order to allow operators to perform their missions with success. On the other hand when it comes to domestic counter terrorism and land based assaults, operators can take advantage of a variety of weapons and equipment that excel in that environment that would otherwise cease to function in harsher environments such as subsurface approaches, over the beach infiltrations or even during mountain and arctic warfare missions.