Agreed, for the most part. What would be more valuable to me though when someone feels that something has improved their listening experience, such as a cable, that it could be tested on their actual system with their music playing. I'm inclined to think neither that all types of feelings about equipment changes are placebo or are not, but a mixture of both. I think all the common arguments have some validity, from expensive systems being able to deliver fantastic enjoyment to ideas that they are grossly over-priced. Where one's beliefs sit within those extremes is up to the individual, primarily because people have different goals when seeking to buy things. I'm somewhere in between.
A wise answer. Considering your role, you are forced into being all things to all people.
Actually, I'm not forced to be anything. More so I've had the chance to be exposed to many different ideas, even before I got back into audio gear in a big way, and exposed to a lot of equipment. Like other people I know who have been equally fortunate, I tend not to hold black-and-white views of things. If I do have strong opinions, the reasons behind them are quite complex.
Your point about medical science was a good one and reminded me that the more complex the topic, the harder it is to nail anything down about it (hence the quote from my friend). With audio, about as far as I've gotten so far is that the most important things are: What music we like and how loud we listen. Just about everything relevant to us derives from that.