I don't agree when you say that uke is unbalanced while attacking.
Well, I think we are just clashing in pet-peeves then. I have had people walk up to me - in range for me to just blast them in the face, stand all totally balanced, then slowly bring their hand up to do a shomen at my head and then wonder why I have to completely change my position to do an ikkyo like thing instead of a normal shomenuchi ikkyo.
There is no martial honesty in ignoring the fact that you have just walked into the range of the person you want to attack and they are suppsoed to not take advantage of that because it's your turn to be the attacker and their turn to be the responder.
Anyway, once we decide it is a bit pointless/fruitless to train that way, we are left with more dynamic situations, and that's really where I am hoping to explain my point of view to you. The way I see it is that I move forward and change the angles (so that they are not 100% in my center vision) and take them in as they attack. Certainly they have to move from a state of "total balance" to "dynamic balance" if they want to hit me or grab me period since I am not suffering from narcalepsy. The way I understand "dynamic balance" is that they are losing it a bit and recovering it quickly. In fact, I believe O-sensei made some statements to that effect like 'I am not centered all the time, I just recover it more quickly than the attackers' - although I think the source is word of mouth from Saotome sensei and relayed from his students (coming from people who teach entire seminars in how "center is balance"). I train to take advantage of those moments, and I claim that you are supposed to do that in aikido and probably judo and karate as well. Otherwise, how do you ever get into the proper position?
One person told me along time ago that in Aikido shite's job is balance and uke's job is unbalance. As I've progressed through the years I have discovered I totally disagree with that statement. I believe that both shite and uke should try to be balanced, but it is shite's job to extend uke so they become unbalanced.
We are totally on the same page regarding this. I think the people making that statement were maybe trying to oversimplfy things. My teacher tells people to always keep your hands in the center-plane of your body, and then proceeds to do techniques where he violates that rule - because that's how he teaches. The point is that I think you have to teach to some basic level, and let the people with more experienced eyes see what is appropriate for them without confusing the majority of the class. He confirms that by walking around and showing and explaining more individually to those who he thinks wouldn't get confused.