OP asked if a hood results in a loss of light that would impact the exposure setting, making reference to the large hood on the 70-200/2.8 II.
The consensus was no, but it does reduce unwanted light (i.e. flare).
Everyone agreed that hoods were good things, and recommended using them.
NotABunny indicated trying a hood on a 70-200/4 IS, and at 200mm it made no difference in the shot, i.e. no reduction in flare.Now...why was that the case? For the reasons previously stated, i.e. zoom lens hoods are designed to not vignette at the long end of the zoom, and as a consequence, they do not protect from flare at the long end of the zoom.
Then TexPhoto made an assertion that it makes no sense to say that hoods protect from flare only/mostly at the wide end of a zoom lens. The most straightforward interpretation of that statement is that you're saying the corollary is true - i.e., if it's wrong to say hoods protect only at the wide end, then they must protect throughout the zoom range. That
was the point of contention.
Now, in retrospect, you may have meant that it makes no sense to say that, becuase it might be construed as a reason not to use a hood at all. Clearly, neither you nor anyone else in this rambling thread is saying that (I hope!).
To correct a few other minor points, it's flare, not flair. Flair is a distinctive elegance or style, something you almost always want
in your photography, vs. flare, which you usually want to eliminate (unless it's used for artistic effect, in which case maybe the flare is contributing to your flair).
Also, the EW-83K hood (for the 24/1.4L II) is a petal-type hood with the ends of the petals rounded off.
But yes, most are squared off a bit, I expect as a design compromise for the convenience of being able to set the lens front-down on it's hood to facilitate changing lenses, etc.
My advice: use the hood (and maybe a gobo). Lots of other peoples advice here: "So, hoods protect mostly/only at the wide end of zooms." Or some longer version of that. Which does not seem too encouraging to the OP.
Again, 'use a hood' is everyone's advice, as far as I can tell. If someone stated that the Canon front lens caps are easier to knock loose because of the side-pinch design, would you construe that as advice against using a lens cap? 'Hoods protect mostly/only at the wide end of zooms' is a statement of fact...not advice against using a hood.