I do this all the time.
Take a greyscale image and you can cut out levels, drop the floor, raise the cieling, etc; it’s VERY useful. If you are using a noise-based alpha, you can expand the range above 1 or below 0 and then saturate to gain/loose the impact of the lows or the highs. More extreme values + saturate are a less-expensive cheap-contrast, which essentially does the same thing anyways.
Single greyscale image can be used to fake-depth on clouds, reactive to the lighting vector and other nifty tricks:
