It’s a taken that the point of a lens hood is to help avoid glare in bright light situations such as where great amounts of direct sunlight is present, especially at the further reaches of telescopic zoom on a zoom lens. Conventional thinking might suggest that the greater the shade provided by the hood the better, especially if you slap the word pro on it (as in short for the word professional and not prostitute) and make it full circular instead of flower/petal shaped?  It might be true in some cases for certain types of pictures, but not all. 


Are we forgetting that wide angle view lenses happened for cameras?

There's a reason that petal shade hoods were specially designed (with a special shape that wasn't just circular) long after the more conventional circular hood. It was to help allow the photographer take wide angle view images using a wide angle view lenses without getting unwanted/unintentional corner clipping/vagination. 

The way that light enters the lens differs from lens to lens, and usually the particular size and shape of a petal hood has been specially designed for a particular model of wide angle view lens. This is to allow for the optimum amount of shade possible whilst avoiding unintentional/unwanted image corner clipping/vagination. This concurs with conventional thinking, like at the times when some of us might have thought "if only I could cut this lens hood a certain way to get rid of the corner clipping in these pictures I take at full zoom". Then eventually science an mathematics stepped in to refine the optimum dimensions for tailored petal shaped lens hoods for certain models of wide angle view lenses.  

The petal shape of the shade has been designed to allow light to enter the wide angle lens in a way that helps to eliminate unwanted/unintentional image corner clipping/vagination when using a wide angle, particularly useful when using a wide angle lens at full telescopic zoom for clearer, none ubscured corner to corner images.  Using a standard circular hood with a wide angle lens at full zoom will more often then not causes the circular hood to clip the corners of the image when using a wide angle view lens. This is why petal lens hoods were designed and used for wide angle lenses.


 
Deep circular hoods will obviously be a lot more effective at helping to avoid /reduce unwanted lens glare, however depending on the type of wide angle lens you're using, certain types of deep circular lens hoods tend to limit the none corner clipping zoom scope of a wide angle view lens. Using a more shallow circular lens hood of the appropriate dimensions would help to negate the corner clipping in wide angle pictures at full zoom, but also at the sacrifice of not getting optimal shade for clearer images. Using an optimally tailored designed petal hood for a specific lens where possible helps to get round this by extending maximum shade cover without obscuring any part of the wide angle viewing area. This is done by creating cut outs in the exact appropriate places in order to avoid blocking the corners of the wide angle lens field of view, especially at full zoom. Hence we end up with the petal hood design. This might seem obvious to some if not many, but it also depends at what angle the main source of light is coming from too, so a bit of common sense will also help to reduce image glare in situations where the hood might be ineffective due to light source positioning.