Again we dont know how OFTEN this happened unless you have evidence? From what I was told this was only in the later years in Iwama.
Well of course, there were periods of time when Saito was out working - but I've been given to understand that it was fairly common. Then again, given Ueshiba's teaching style I wonder how much actual hands on there was for the average student even when he was actually the active instructor. Saito's main advantage was as a crash test dummy during Ueshiba's personal training and research.