Well...that's the thing. It's an important part of Japanese culture, and it has correspondingly become a part of Aikido culture.
But that doesn't necessarily mean it's important any more than we should be eating Natto and drinking green tea just because Doshu does (I recommend both, but that's just my personal preference).
The question is, why is it necessary and what are the drawbacks? Especially, what are the drawbacks of implanting it in a foreign culture, and are the benefits really worth it?
IMO - the early instructors coming out of Japan (Tamura, Tada, Chiba, Yamada, Kanai, etc.) had gotten most of their actual teaching experience in university clubs during the build up after the war.
University clubs are notorious for their sempai-kohai shenanigans, and much of that was brought with them out of Japan.The sempai-kohai system was a familiar method for young instructors to impose order in a foreign environment. It might have been different if things had developed more organically.
I also wouldn't discount the effect of pre-war militarism on the martial arts - evidenced in the difference between koryu and gendai practices.
Na, the comparison with food and tea is over-simplifying the matter.
In the Western society we already have a junior-senior structure - but most people just do not seem to be aware of it. Perhaps because in daily life we no longer have it in a formal structure, but it is nevertheless there.
By practicing sempai - kohai structure (to me it is part of keiko) in the dojo one becomes conscious of it. And this may have a positive effect in the way we help and support one another.
I remember several training sessions by Tamura sensei were he explained sempai - kohai as a way to support each other. There was no emphasis on hierarchy as such. This can also be found in his books.
The problems that I have seen in a number of Aikido dojo were a result of NOT understanding sempai-kohai relation, and with that came often also a not understanding of the real relationship between teacher and student. Instead of a sempai-kohai structure they had an ordinary pecking order.
As far as the university shenanigans - don't you have them in the US as well? They do happen in the Netherlands, UK and France and they are usually between senior students and newcomers. And it has been like that for a long time.
It may or may not have had an effect on their teachings in the beginning, I am not aware of that.
I would not discount the effect of pre-war militarism either - I think it is important that everyone who practices Japanese Budo should educate themselves about the history and become also aware of the dark side of Budo.
At the same time I feel that in that period all over the world a lot of things that were in itself innocent and even good were stolen from us and misused.