I have used them. We had a system that generated millions of images and needed to be sure that from one version to the next the images produced by a given requ

I have used them. We had a system that generated millions of images and needed to be sure that from one version to the next the images produced by a given request were the same, and also have some diagnostic data in case of problematic images. The images could be either JPG or PNG and we needed a unified way to associate arbitrary metadata with them. We had a special mode that would store an equivalent of the request in an alternate data stream of the image. When a problem was detected we would open the alternate data stream and test the request manually.