They’re super useful once you get used to them. No matter your weight, if you fly at the right AOA you’ll have the right stall margin. It is easier to see the indexer than the ASI, for control on approach. It acts like a pitch rate indicator too, if you are rough with the nose, so it helps to smooth out the flying. Responds much faster than the ASI. If you are maneuvering aggressively, it will give you that stall margin too.
Update with nice paper from AIAA Journal of Aircraft:
"Low-Cost Accurate Angle-of-Attack System", Borja Martos and David F. Rogers, Journal of Aircraft 2018 55:2, 660-665
of attack for maximum lift-to-drag ratio corresponds to maximum range
and maximum glide ratio for a piston engine- propeller aircraft.
Hence, in a fuel critical situation and/or an engine failure
situation, it is critical that the pilot have accurate access to this
angle of attack.
With today’s fuel costs, the Carson cruise  angle of attack, which
represents the most efficient way to fly fast with the least increase
in fuel consumption, is of significant interest.
The angle of attack for maximum lift-to-drag ratio does not depend on
density altitude, weight, or load factor but only on aircraft design
parameters and aircraft configuration. Similarly, the angles of attack
for minimum power required and Carson cruise depend only on aircraft
design parameters and configuration...
Therefore, a low-cost, accurate, full envelope, easy to flight
calibrate angle-of-attack system is a transformative technology for
light general aviation aircraft.