You need one precisely to know where your are, aerodynamically speaking, whatever would be the bank angle, load factor, weight (including ballast), flaps position or airbrake extension... On any case it will show the real position of your wing in the surrounding air, and how it changes, and this is the only thing that matters to fly...

Sorry if you learn that speed is life... AOA gives a much more straightforward information of your real safety margin, or 'lift reserve' before you fall from the sky...

And it indicates directly if you fly at the lowest possible rate of descent or at the best glide angle as these angles are purely aerodynamic parameters that does not depend on anything else.

The only thing that will have a slight influence is a modification of the airfoil in front of the separation point, like the extension of a leading edge slat.

Flaps have no influence on stall AOA, as the separation point is before the flap along the airfoil, but as it changes the camber of the airfoil, it changes its maximum lift... reducing the stall speed when positive, and allowing to keep the best angle through an extended speed range when negative... One more reason why speed isn't safety...

Of course, you can compute the stall or best speed form the mass of the glider and actual load factor, flaps position, airbrake position, and compare this computed speed with what your ASI indicates, but presently no instrument does this for you accurately and transparently.

The AOA indicator will be accurate as long as the measured pressures are significant. In a spin, It will indicate that you are above stall... Maybe the exact angle can't be determined as the dynamic pressure is likely to be inaccurate due to the airflow angle Vs the probe, but as soon as you recover the indication will be back instantly...

And this is precisely a situation where it is useful: When you recover from a dive, you will pull at least 2-3 g... You only have your feeling to pull enough to recover and stay below the Vne, but not too much or you risk a high speed stall... With an AOA, you can adjust precisely how much you pull the stick to maintain the indicator in the good range. This will guarantee that you will lose the minimum of altitude to recover from the dive...

The instrument is based on a pressure ratio. It's easy to understand that if the angle difference is low, the pressures will have less difference and the ratio begins to be inaccurate. So the ideal angle is 45°... But this is not common on gliders. AOA probes on fighters have about a 20° angle and they work even in supersonic airflow... So I would say that 20° is a reasonable minimum, and this can be found under the nose of any glider... And this is a place where the flow has no reason to be disturbed or modified within the allowed speed range, whatever will be the camber and shape of the nose.

Then as you calibrate the instrument for the different angles in flight, by pushing a button when you are stabilized at each reference angle, there is no further adjustment needed...

Any other question?