I'm sorry I upset you so much, and I apologize for misunderstanding what you were saying. I did assume that you were talking about speeding (above the limit) down hill, and I do now see that you didn't say that. That takes the wind out of my "unsafe and illegal" points. I could still find ways, by stretching a point, to argue for them, but I know I couldn't convince you.
I will, however, continue to argue for two points I made. (As in all things, I am willing and able to be convinced of my errors, so don't give up on me.)
Point 1: Coasting is either not 100% efficient, or is pointless in contrast to using regen. If a downhill slope is so gradual that coasting does not speed you up, then you don't gain any kinetic energy from it and you wouldn't gain any electrical energy from regen. That is the pointless case. If the slope is steep enough to speed you up, then the higher speed causes you to lose additional energy from air resistance, energy that you would not lose if you held your speed constant, as you would with regen. Clearly a loss of efficiency. (Again, regen also has efficiency losses. I'm only saying that neither is 100% efficient.)
Point 2: Coasting, as practiced on a through road without speed impediments, is antisocial. Now, antisocial doesn't matter if you aren't interacting with other people, so I have no objection to how unevenly you drive if you are the only car on the road. But I do object strongly to trying to follow someone who is constantly changing their speed. And I also object if I decide to go around them while they are going slowly, and then they speed up as I attempt to pass. I call those behaviors antisocial because you are making it more difficult for others to share the road with you. Also, many of our highways are heavily traveled, and the most efficient way to use them is for all cars in a line to move at a steady speed. Speeding and slowing can cause traffic jams, and that is highly antisocial.
Finally, in my defense, let me say that, just as I have misjudged you, you have also misjudged me as a driver. I normally drive in the right lane of a freeway at a constant 60 mph when the traffic allows. When I see slow traffic ahead I tap my brakes twice and then use engine braking (ICE) or regen (Prius) to reduce my speed if feasible. On city streets I watch traffic signals as far ahead as I can and try to coast up to red lights, with a goal of not having to stop completely at all.