So far, what I've read about denoising is always the context of doing image post-processing, but it seems to me that some of these techniques could be used just as well to identify areas of the image that the denoiser is most uncertain about, so that you can trace more rays in those directions.
Sure if you want to reduce error at some cost, you can use a noise metric to identify where you should send more rays. The premise of denoising is that it's cheaper and you've already spent enough time on the analytical algorithms. Also there is a chance that the noise/variance is due to a high variance feature which (a) would have been fine to leave out and (b) causes a cascade of "noise-driven" ray tracing.
