While overall a less efficient way to render, progressive rendering is useful when you want to get as many samples as you can in a certain amount of time, without having to estimate the number of samples you will need. You can just set the samples to an absurd number, then let it go until you need to stop or are willing to call it good enough, which will net you with a full size image.
The problem however, is that to my knowledge there does not exist a way to stop a progressive render and then have it then be denoised at whatever level of quality it reached. As it stands, if you want to render progressively for an arbitrary amount of time you must sacrifice denoising.
I wonder if when using progressive rendering there could be something like a “finish rendering” button?
or maybe a tick box that says “process after cancel” or something like that?