Good to see the discussion of the FPV, whether on a HUD or on the flat panels.
My point is that the sucker shows actual flight path regardless of the air data. Surely this would have allowed the pilot to see that he was climbing and not simply maintaining the existing flight path when the air data went FUBAR, and the system reverted to mode "b", sub-mode"III, and so forth.
As others have pointed out, flying in IMC or in pitch black darkness, that FPV related to the horizon line is very comforting. Talk to any nasal radiator that flew Scooters or Double Uglies off the boat at night. No visual references of any kind. Acceleration from the cat shot that forced you to fly instruments. The exchange pilots I flew with that went into SLUF's, Hornets or Tomcats will tell you that the HUD flight path marker and that horizon line were lifesavers. And then I had my own air data failure one day in IMC and did just fine using actual flight path data than attitude with no air data. The other thingie we had was inertial vertical velocity next to the altitude bar. Think that doofer would have helped the pilot figuring out he was climbing like a bat outta hell or finally descending after arcing ov er the top with very low speed and high AoA that he could not determine from the flat panels?
I realize that some here will re-state the requirement for our basic airmanship and instrument flying skills. We always emphazised the ability to fly using the steam gauges. But I gotta tellya that comparing the two types of data - inertial and air data, was the best crosscheck ever invented.