This is the second, but most complete stage of experimentation I've done with the use of shampoos and soaps and has completely satisfied my curiosity in regards to their uselessness. I'll share my findings for all those interested.
Beginning in February I decided to run an experiment wherein I would forgo any use of soaps or shampoos and study the results. My suspicion was that these products create their own demand through their continued use. My hypothesis, based on past experience and research, was that it is the very use of soaps and shampoos that causes our bodies to require the use of them, and that being vegan, with a thoroughly clean system, there is no need for the use of soaps.
Prior to giving up soaps/shampoos, my skin was often dry and flaky, my scalp would get occasionally itchy and I would have dandruff (not heavy, but noticeable, especially when scratching the itch). The hair on my head following shampoo use would be dry and dead/limp, and after a day or two without shampooing it would become quite greasy.
My theory was that these symptoms were all brought on by the use of shampoos and soaps, and that the greasiness of my hair after a few days of not using shampoos/soaps was the result of my body's attempt to recover from the damage done by the shampoos, but that it would regain natural balance over greater time.
I've tried many brands and types of soap over the years, but had settled into using Doctor Bronner's products... seeking as 'natural' a product as possible. I'm aware of many alternative products, but my belief is that the results of my experiment would be similar for any such product. My previous experiment was with standard products and yielded almost identical results (though at that time I was vegetarian, not vegan, and the results were not as optimal).
So, to my current results. During the first week to two weeks my skin continued to be dry and flaky and my hair was almost unmanageably greasy. I spent more time in the shower than usual, scrubbing with my fingers under hot water to try to 'rinse the grease away' before going to work (as a server)... but that did nothing to alleviate the condition. However, after about a month, the grease levels balanced out naturally and now my hair feels clean and soft to the touch and no longer becomes greasy. My skin is no longer dry; my hair no longer itches, and I no longer get dandruff. On all accounts, my hairs is as healthy as it has ever been. It also appears relatively thicker, though I think this may be due to the thickness of the hairs as opposed to new hairs having grown in. I'll continue to keep an eye on that over time, though.
My conclusion is indeed that the use of shampoo strips our hair such that our bodies react to it, causing the conditions that lead us to desire the use of shampoo. My conclusion for soap is the same. I have noticed no increase in 'dirtiness', greasiness, etc., of my skin (in any sense), and being vegan I have little to no noticeable body odor. Both products appear, to me, to be completely useless.
So, here's my testimony, from experimentation, that I believe these products are unnecessary, and, in fact, are a sort of self-fulfilling product - both the cause of and 'solution' to the conditions requiring their use. As someone who appreciates testimony as much as data or scientific evidence, I hope this helps others on their journeys. There are several discussions on 30BaD with both testimonials and science on these topics, if anyone wishes to run a search for them.
Here's a starter search to get you going.