I think that offering resistance as uke is not only a good thing, but a necessary thing. However what I see as a problem is the attitude of "helping them to see what they are doing wrong". This way of thinking is very egotistical, arrogant and can hinder growth and learning. Reflecting on my past experiences when I attempted to "correct or help" tori, I see now that it was I who did not understand what they were trying to do. Even people who are kohai may see small details in techniques and a lack of vision coupled with arrogance can be harmful. In my experience as tori, the biggest obstacles to my learning has been all the teaching.
Mr. Stefan commented:
"But it is important not to resist the particular technique practiced at the moment - because that is almost pointless, and an aiki mood would really be for tori to immediately switch to another technique."
I don't agree with that. I think that if uke resists and tori switches to another technique then tori is missing a chance for growth and learning by avoiding the problem.
You also stated that "Another way of polishing one's techniques is in kaeshiwaza, countering techniques. A properly done aikido technique should really not be possible to counter - but that's easier said than done."
I completely agree that a properly done aikido technique cannot be countered, and that a good way of polishing one's techniques can be through the use of kaeshiwaza. However, the attacker must still provide the initial attack with resistance. If he just remains passive and waits to counter tori's technique, then isn't he in fact...tori?