Does the Ambient Occlusion material input have any special effect other than simply darkening the texture? For example, is it used for displaying reflections based on the AO map or something similar, or should the input be ignored and the AO texture just be applied to the diffuse texture in Photoshop?
it darkens the ambient contribution of lighting, it does not darken direct lighting (same as SSAO).This way it doesn’t appear as dirt when you light up an object with a lightsource. AO in photoshop will have the appearance of dirt/smoked.