
The light he's talking about is still common today, and it's hardly unique to Paris, but in Paris there are many nice things to look at and appreciate when light of this type is available. It's essentially just due to very even, thin overcasts that diffuse sunlight into an almost perfectly uniform white sky, without substantially reducing the intensity of the light. So you get a very bright, shadowless, perfectly even illumination that can be ideal for some types of photography. You may not need any artificial light at all. If you just want to light your scenes in a utilitarian, effective, and inconspicious way (as Frankenheimer would have wanted for Ronin), this sort of weather is ideal.

The main thing that is nice about Paris weather, compared to the desert hellhole in which I was born and raised, is that it is highly variable without being extreme. Some days it's sunny, other days it's overcast. Sometimes it rains, sometimes it doesn't. Sometimes it's chilly, sometimes it's warm (I prefer chilly). In my original hometown, every single day of the year (with a handful of exceptions) was sunny, hot, and cloudless. That kind of invariable weather has some practical advantages, but it gets really old after a while, and when the heat is oppressive it becomes positively stressful.
Anyway, I walked around recently on one of these John-Frankenheimer days, still wearing sunglasses because the overcast was extremely thin, and I thought of his comments again. It also made me think of many video games, in which there's a kind of magic, all-around light without shadows that clearly illuminates everything; some of the games I like (The Sims, Second Life) are like this.
While walking around, I saw a bicycle tour riding on the sidewalk, which isn't strictly legal. It's usually tolerated because being on the sidewalk avoids vehicular traffic, but when you have a dozen or more people on bicycles riding on the sidewalk simultaneously, it's a substantial hazard to pedestrians. I have to wonder why the tour leader chose to do this, as it's not very safe.
Just a short time after that, I saw another tour (by the same company) on Segways, near the Invalides. Segways are legally considered pedestrians, unlike bicycles. The irony is that sometimes the tours have people riding bicycles on the sidewalks, and sometimes they have people riding Segways on the street. Oh, well … no accidents thus far, as far as I know.