Do pro photographers use "Auto Lighting Optimizer"? Is it any good? If the histogram works well, why should we use ALO?
"Auto Lighting Optimizer" is Canon's term for in-camera post-processing to automatically adjust shadows and highlights in high-contrast images. Nikon has a similar feature called " "Active D-Lighting", and other makers have corresponding capabilities as well.
These all do the same thing: they adjust the tone curve so shadows are brightened and highlights roll off more smoothly. The exact function is "secret sauce" — you don't get much control over the exact effect. It's very much like using a software program which can adjust this curve and hitting the "auto" button. Sometimes you get good results, and sometimes... not so much.
The histogram is basically unrelated, although if you see that your scene smashes up against both ends, that's a good indication that using this feature might help.
Now, as for whether "pro photographers" use this: sure. Some definitely do. Professional photographers do many different things, have many different approaches, and correspondingly have very different needs. This doesn't have very much impact on whether the feature is good for you, even if you are a pro yourself or aspire to be one.
For some pros, getting decent results fast is the most important. They're trying to make a living, not art, or perfection. Shooting in JPEG and using the camera's JPEG capabilities to the fullest is very convenient here. For others, meticulous control is important; these pros shoot in RAW and hand-tune everything. Others fall into the spectrum in between.
I'm not a professional photographer, but I'd be fairly sure the answer to this one is "no" as Auto Lighting Optimizer has an effect only if you're shooting JPEG or processing with Digital Photo Professional. As the workflow for the majority of professional photographers probably don't do either of those, Auto Lighting Optimizer is a dead option for them.
For what it's worth, auto lighting optimizer and the histogram have different uses - while mostly all you can do with the histogram is to translate it left or right by changing the exposure, auto lighting optimizer changes the tone curve that is applied to the raw data, so actually changes the shape of the histogram as well as its position.
I use ALO on normal on my camera, but only for the on-camera preview. I haven't verified if it makes a difference on the camera preview with any definitive test, but it was the default setting and I never found a reason to change it since it doesn't impact the RAW files themselves.
I don't check preview images often, but when I do, I like it if they are closer to what my final color corrections will actually be. I do wedding photography part time as a side job.