I think NagaBaba is correct in detailing the benefits of having a resistant Uke, but both the OP and her troublesome Uke are both mid-range kyudansha. At that stage it is easy to thwart the learning process of Nage - Uke knows the attack and the technique. I think it preferable to lighten up and allow Nage to develop the gross movements without excessive resistance. I define excessive resistance is that resistance that is so overwhelming Nage is stymied. I often ask my partner, regardless of rank, to increase resistance as we go along and I find that helpful for my own training. If I get stymied then, I have to find a way within myself to make it work without resorting to henka waza.
I don't believe there is something special with mid-range kyudansha. Everybody, regardless his level, has to face the situation where his present abilities are far from sufficient to deal with the attack. Immediately, your ego stop growing, and you are finding yourself in ‘right place'. I'd say, it is true even more for high ranking folks.
Second process that is starting in your head is --" all what I've learned until today is not good enough to deal with THIS! -> here we have 2 possibilities
1. People quite practice
2. Such person changes completely his approach to training, to incorporate ‘new' (new for him) aspect of art.
It plays a very similar role to Koan in Zen training. Without hitting THE wall with no results, the breakthrough is not possible. Your progressive approach also will not provide desired results at this level.