Ideally, yes, no opinions, only empty mind. This way they have no emotional or intellectual filters, and can learn exactly what instructor is teaching. Have you ever seen such situation, when during seminar guest instructor is trying to teach something, but 99% ppl still practice exactly the same way as they do in their own dojo? That happens because they already have their own opinion how the technique should work.
Regarding your second question, I have few friends here and I like to chat with them sometimes. Also there are some very advanced aikidoka whose opinions are important to read.
I'm not against discussing Aikido, but against no-kyu and 5th kyu shihans lecturing everybody what aikido is and how we should practice. They create the informational noise on the forum that suffocates valuable info that comes from experienced folks.
5th kyu and no Kyu's, hopefully, come to Aikido with an open mind or at least have fewer Aikido habits built up. This is why, they should be able to have some voice, not a lot, but some voice.
I am 5th Kyu and recently went to a seminar and what you described while not common did occur (the Sensei complimented the dojo for it's openness to her instruction). It in fact happened to one of my partners. We were doing a connectivity exercise where I was supposed to resist/push back when he moved his hands towards me and when he felt the pressure he was supposed to stop the forward motion. As instructed when his hands moved forward I resisted/pushed back and he being stronger pushed through my resistance. I asked and he said he was just pushing through and doing the technique properly. He's a black belt so my assumption was that I had misunderstood the exercise and we continued on in that manner until corrected by the Sensei. I got chastised for not resisting until the instructor realized the situation and then corrected the balckbelt. If the blackbelt was open minded and willing to listen
either to the instructor or the 5th kyu then more time would have been spent learning from the exercise.
That is not to say that out of babes comes wisdom. I have 3 kids all under the age of 8. Most of the time what they have to say is feed me, I don't want to and now that the boys are a little older really silly fart jokes. My house is filled with noise and I have a full fledged 7 year old Shihan well versed in the ways of Mythbusters. But if I listen, occasionally there is insight. Now, I don't let them lecture me but I am willing to listen when they have something to contribute.
When I first saw this thread and read the original post. I dismissed it. It felt like so many of the blogs and message board posts that are out on the internet. The beginner having a break through and insights wanting to share their experience. I find them a bit self aggrandizing but that is perhaps because I grew up in a Methodist church and we didn't testify! There is nothing wrong with this and perhaps others get more from them than I do. I keep a journal of my insight and learned technique. I keep them to myself (in general) since well they are mine and to be honest I don't have enough data to publish and as of yet have not had any insight that I have not seen expressed else where.
But I still look to see what others have to say. Sometimes there's something there and to chastise them for their efforts seems to defeat the opportunity. I also see no harm in beginners hashing out there ideas and insight among themselves. Tell each other what they are experiencing and learning. It shouldn't detract from getting on the mat. And it shouldn't be taken as great insight. But dialogue can help some people understand better.