Why appeal to the age of an authority? If there's anything measurably beneficial about Ayahuasca I'm certain it can be extracted into a precisely dosed pharmaceutical.
The danger of your opinion is that it precludes research into things like drumming and chanting. Humans have been using rhythm to build a sense of community for thousands of years. Could there be something here for science? Could we learn how to help therapists and patients bond much faster?
I don't know these answers. Do you?
The shaman by him/herself won't do anything. Neither would the Ayahuasca on it's own. It's having the right combination of both (setting), and your intention (set) that leads to an amazing experience.
The funny thing is, the experience itself is not the most important thing.. it's what you do after the ceremony with your life, and how you change your lifestyle to make you happy.
Obviously, if the shaman isn't giving you the real tea, mixes stuff in, doesn't take care of you, and such, I'd call it a straight up scam.
If you like, you can think of it as a kind of placebo effect. Which, you may be able to imagine, can get really strong under the influence of psychedelics.
Disclaimer: I've never used Ayahuasca, but I did organise a few shroom trips, where I paid a lot of attention to the "set & setting". Compared to other times, it makes a lot of difference.