From my previous question, I was wondering what the alpha sliders are useful for in cycles. (As they don't seem to effect the shader)
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I think it's safe to say that it does nothing at all. It's only present there because all shaders use the standard RGBA selector even though an RGB selector would be sufficient.
In some other areas where only RGB-values make sense, however, RGB-selectors are in use, so it's not great user communication to not use them there I guess.