So, we call them hazard lights, right? Not "I'm-stopped-on-the-side-of-the-road-lights".
I'd say that driving unusually slow, whether up a hill or down a hill or dealing with reduced visibility or a mechanical issue, or delivering the mail with stop and go driving, or a farmer driving a tractor on a state road...you are a hazard.
It makes awfully good sense to me to let other drivers know- "hey, I'm driving unusually" That's exactly what those blinking lights convey!
I don't care what the law says- the law and optimum safety are not the same thing- I appreciate someone who's 15 or 20 MPH below the speed limit giving me a heads up as I approach them, I appreciate it when a traffic jam begins to form, and I appreciate it when someone hits a sudden downpour.
I do worry at these folks who blurt out that it's some kind of hazard to run your hazard lights. Informing other drivers that you are not going with the flow is a very safe thing to do.
Now, as to the question of reduced visibility where everyone has slowed down to the same slower speed???...maybe then they don't add much then, but then again, I call bologna that they cause harm.