elephant I do fear that you are massively over generalising. Neither do I think a school lesson has a remotely similar purpose to a university open day.
Yes some parents are overly pushy, and I can recollect a couple of cringe inducing questions at the Open Days I had the temerity to attend with DS1 when he was going through the application process. However in the main they, parent(s), are there to listen, act as sounding boards - presumably you wouldn't question the value of mentoring and coaching in the business world. Largely, rather like Victorian school children, parents should perhaps be seen but not heard at open days. I would argue that on accommodation, usually informal tours, parents do often have valid questions to ask that their DC often would not even thought about. And having a younger DC available can be useful to demonstrate how tiny and of limited use some en-suites in Uni halls are :)