You are right, it's the rider's prerogative to use flashing lights and high-vis in the daytime. There are no laws requiring such things. But I have PERSONALLY EXPERIENCED how much easier/quicker it is to see/notice cyclists from my car when they are using flashing lights and/or high-vis clothing while I'm in my car. I was driving along and saw a flashing light in the distance. I noticed it because it was flashing. "Oh cool someone on a bike" I think, since you hardly ever see anyone on bikes around here. And because I noticed that flashing light from a long distance away, I was more aware of his presence and ready to maneuver properly as I approached him, with plenty of time to spare. Without the flashing light, I likely would not have noticed the guy until I was much closer.

And as [MENTION=36008]Leisesturm[/MENTION] said above, cars are pretty visible. But white cars are the MOST visible. They stand out from the crowd much more than other car colors. Same with the daytime running lights, especially in shady areas.

I can tell you that if you're riding on a road with lots of shade or lots of alternating shade/sun (roads with lots of overhanging trees), and you're wearing darker or drab-color clothing, you'll be nearly invisible to most drivers from more than a few hundred feet, for those sections while you're in the shade. That's especially true if you're in the shade in the foreground, and farther down the road is a brighter area. The contrast between the bright/shade areas makes it that much harder to make out details of objects in the shade. Flashing lights and high-vis help greatly in that respect.