...the primary reason HUD is considered essential in fighters...
The military fighter HUD's primary function is to aid shooting AT or bombing some poor disadvantaged adversary. If you become the 'shootee' instead of the 'shooter', a military HUD itself at that moment is of reduced value.
Military-wise, HUD instrument approach capability has always been secondary, and in fact in the early small HUD F-16, when you got to the 'rabbit' in thick, wet fog or snow, IMO you were better off going true heads down on the instruments due to the disorienting reflections and glare off the big curving scenic canopy. Even better was to have the sequenced flashers turned off when possible, something you generally realized after the fact. But I guess HUD is really valuable around the boat.
Civil aviation HUDs (or now more correctly HGS), though providing a range of functions from the mundane to the critical, are optimized for low visibility instrument approach capability. A couple of operators have already added IR capability, gaining a credit which effectively gets you even lower minima.
However, constantly flying nothing but HUD can also create a new species of technological dependents, the 'HUD cripples'. HGS, as a system, or components can fail...
So there we are again. That said, I personally would rather have one than not.
