I have the hood for the CV 35/2.5 and 50/2.5 but I admit that I don't use it much. With my Bessa, I generally carry the camera with a neck strap under my right arm with its bag back at the car/house/etc. Without the hood, the whole package is so handy and comfortable to carry that I'm more likely to carry it. (Side grip attached, of course.) For these lenses, the dinky ring "hood" offers good protection and the font elements of these lenses aren't very protuberant.
With my ZM 50/2, ZM 35/2 and my Ikon, I always have the hood on the lens that I am using as protection for the front element. I usually carry this camera with a wrist strap and its bag over a shoulder. In my hand, I feel the lens might be more likely to inadvertently bounce off stuff. The front elements on these ZM lenses are larger and seem more exposed when the hood isn't attached.
Flare-wise, I've only had a few occasions that have ruined photos. One occurred with the CV 50/2.5 in near-blinding midsummer sun near the horizon. I should have put the hood on the Bessa that day.
Of course, flare resistance varies with lenses. My collapsible Summicron 50/2 doesn't leave the house without a hood. Compared to my other RF lenses, it seems to be a flare magnet.
The round vented hoods tend to be larger and are vented to lessen the portion of the viewfinder blocked by the hood. The smaller rectangular must be positioned so as not to block the viewfinder. The larger of these have a port to minimize blockage of the viewfinder. Unvented round metal or rubber hoods block the viewfinder the most and consequently I don't use them with 35 mm rangefinders.