I often use the VFB for outputting final frames so I can see what's going on. Many times I wish there was some means to see which buckets have not yet rendered, particularly when there is a lot of empty space/transparency in the frame. It's useful to be able to "clean up" these buckets as quickly as possible to force the adaptive subdivision to kick in, and often to avoid letting the renderer pick up certain areas of an image until late in the render for that reason. Currently there's no way to see this that I'm aware of.