This topic draws inconclusive and no amount of statistics will justify it. Individual difference applied here as much as it is to other stuff like helmets hi viz and x y z. It works for some and not others. I'm sure we have all heard of the person who crashed because they was fiddling with the sound controls in the car, hence why they started using volume controls on the steering wheel.
Like many of the 'advances' in car design over the past few decades, I am not convinced that this was safety-driven.
There were other factors too, not least sales and the difficulty of fitting non-OE parts.
I agree with the first part of your post.... It works for some and not for others.
Have a look at drivers... Many people don't use them. The last five cars we've owned have had audio controls on the wheel... initially volume, but now multi-fuction for changing station, levels, getting into and out of CD, MP3, radio et al... And these are not super-posho cars.
Do we use them? Maybe the volume occasionally, but not the others. We lean down and use the buttons on the stereo. So do very many middle-aged drivers.
It looks cool in the showroom. I do not think it was just a safety-related decision to fit these things.