What kind of evidence are you looking for? Anecdotal is all your going to get obviously. I'm married, and my wife can't tell the difference in smell except there is no shampoo smell. People putting their nose in your head and taking a big whiff and comparing that to before is about all you can do. Same with skin. You are better able to smell your skin, but the same thing applies. Last time I took the shears to my hair I kept some to smell, and it is just no smelly smell at all. Likewise many, if not most, do encounter noticeable hair problems when first going shampooless, and it follows a pretty predictable problem of increased oil. The more product the one has been using, the more likely the problems will be extended.

There are other things like this as well. I used to use shaving cream when I shaved, but I always just shave dry now no problem. But when I first was doing it, it was a little more difficult/rough on my skin. (Like what is explained here: http://www.lewrockwell.com/tucker/tucker65.html)