There's a fairly narrow canyon in my neighborhood that sees a fair bit of bicycle traffic. It gets very little direct sunlight, since it runs north to south, with the lower end of the canyon to the north. I've cycled this road a few times and have been driving it fairly regularly for the last few months. A number cyclists have taken to using blinkie taillights when they ride this road, even in daylight, and I'm surprised at how much easier it is to see the cyclists with taillights. And yes, I do watch for cyclists.
In short, there are plenty of situations where a modern LED taillight will make a cyclist easier to see even in broad daylight, even if the motorist is in aware of cyclists on the road and looking out for them.