Say you are shooting an Egret at 1/1000 sec @ f/8, with an ISO of 400. Perfect light and exposure confirmed by following the histogram…. Right?
A cloud comes in and diffuses the light. All I have to do is to reduce the shutter speed to say… 1/500 sec (one f/stop) and continue achieving a good exposure without sacrificing noise by allowing the camera to chance the ISO. If the cloud rolls out, my shutter speed will of course return to 1/1000 sec which I already know is a good
and confirmed exposure based on the previous light.
That’s two or three click to the left and right on the top dial.
My point is that you can easily achieve the same results without sacrificing noise and more importantly…. Image quality.
You could do that, but you have to keep in mind the side effects of changing shutter speed or aperture. Both are indeed used to adjust exposure, but they have distinct side effects. A lower shutter speed will increase blur...both blur from motion as well as blur from camera shake. A wider aperture will reduce your depth of field, potentially to the point where half your subject is out of focus when you wanted the whole thing in focus.
At 1/500th with a 400mm lens, I experience just enough camera shake blur when hand-held for it to blur detail when shooting hand-held, even with IS (admittedly, the IS on my 100-400 isn't a modern 4-stop version). Additionally, 1/500th of a second exposure can cause more wing motion blur than I want in some cases. I liked your 1/30th second shot of the heron, but you don't always need that low of a shutter speed to get a nice, artistic amount of motion blur in wings. A fast moving bird with a fast wing beat at 1/800th - 1/1000th produces enough of blur for my tastes most of the time. In that case, I really want to keep my shutter speed between a pretty narrow range, limiting my ability to adjust it if exposure correction is needed. If I require a specific aperture to achieve enough DOF to get a whole bird, say a large bird like a pelican, in focus while tracking it in flight, the only thing I can vary at that point without worrying I'll impose undesirable side effects is ISO.
I could certainly change ISO manually (and I always do, given that I use a 7D and don't have the option of EC in M), but if I had exposure compensation against Auto ISO in manual mode, that makes the adjustment so much easier. I don't need to drop into a specific camera mode to change ISO to my desired level, exit that mode, then focus/lock and track my subject. With EC, it would simply take the roll of a dial to achieve the exact same thing, but without losing
the instantaneous ability to adapt to changing lighting circumstances. Its definitely a convenience thing...but convenience means I spend more time watching the birds (and getting keepers) and less time watching my settings.