I have to go with Okie on the utility of a flight path vector, not a flight path angle.
The two jets I flew that used the inertial system to display our flight path vector showed actual vector in space, not simply angle above/below the horizon. 'course, this was because we used the system for weapon delivery, so "drift" was automatically compensated for, as well as other parameters. No more TLBAR when bombing ( that looks 'bout right, heh heh).
The basic inertial platform was also used for the "attitude" indicator ( steam gauge). If the accelerometers went TU, then the sucker still worked like any other attitude indicator, and could be re-initialized as with the older systems, just no velocity vector. remember the J-8 from the 60's?
The beauty of our systems was their independence of air data. As long as we had electrons, we had an excellent display to use if the air data was completely gone, and our vertical position in space was excellent. Now, it is true that we used air data to smooth the vertical position in space ( system altitude), but if the air data went TU, no big deal. Inertial drift of the 70's and 80's platforms was less than one knot/hour in all reference axis.
To summarize, using an inertial platform to provide flight path data and speed and altitude is very useful if the pitot-static system goes west. Granted, the plane flies in the air mass, so actual speed thru the airmass is important. But for short periods, it's a life-saver until your air data sensors thaw out. Even if they don't, you can use the rules-of-thumb for power and attitude. Oh yeah, we had AoA sensors that might still be working if the other sensors froze up. Fine for stall avoidance, not so useful for limiting mach parameter.