Yay! Someone is using Layers in A:M!
Thanks for sharing that tip Tore!
With Layers we can also adjust some of the settings to alter the look and feel.
For example: Cranking up the ambiance intensity.
And not that you'd use it in this production but changing the ambiance color and then the intensity of layers can yield some interesting animated effects.
Another neat tricks with layers that your current style makes me think of are things like shadowy figures moving in the foreground or animated vignettes or atmospherics (for instance some noise/haze placed both in front of and in back of the characters in a scene.
And as you say, because they are prerendered they re-render very quickly.
I will add:
A few of the downsides to using Layers vs Patch Images (i.e. images assigned to a single patch ala Layers) is that with Layers only one image can be used. With patches we can also skew and bend the patch into different shapes whereas Layers are confined to a rectangular plane. Where the images are straightforward planar images it's hard to beat a Layer. I'd have to check but I think Layers produce much better shadows (for the light shining through transparent/alpha areas.