I'd agree with George. You need both solo work and partner practice. Of course, what kind of each is still important.
And recently I had some preconceptions shattered that touch on this subject. I've yet to really dig into things, but below is a general overview.
Ueshiba really didn't spend a lot of time training with Takeda.
Shioda, Tomiki, Mochizuki, Shirata really didn't spend a lot of time training with Ueshiba.
With Ueshiba, I think Takeda gave him specific things to train. I think Ueshiba did a lot of solo training, but I think he also trained with other people when not training with Takeda. There was the training with the Oomoto followers, for example.
Regarding Shioda, Tomiki, etc, I think they were training with Ueshiba at a critical time where Ueshiba was still working things out. So, by that very nature, they were exposed to specific exercises and training methods. And they had each other to train with.
But, it is hard to deny that each student spent a lot less time with their teacher than they did training either on their own or with other partners. At some point after learning aiki, it was their responsibility to take it further. And several of Takeda's students were quoted as having said they took aiki further or in a different direction than Takeda.
Note the training listed below. Note how there really isn't any lengthy training periods of student-teacher. Even though there were 4-5 training sessions per day, Ueshiba was not always there. He had a busy schedule and was gone quite a bit. We're looking at 10 years at most for students and even then, it wasn't "extensive" training with Ueshiba. I think people would find it surprising just how many days Ueshiba was gone from his main dojo.
Now add to that all the students stating that he never really explained. That he only showed things once. And when he talked, it was spiritual things that they didn't understand. Sounds a lot like the training in post war.
I think the difference between the two was that pre-war, Ueshiba was still working aiki, building aiki, changing his body with aiki and it showed through in certain ways that his students could pick up on. Post-war, he was beyond that point and what he was working on was more completely internal and couldn't be seen.
The whole point about experience though is that these martial giants didn't need "extensive" training with a teacher being there all the time. They needed "extensive" training with what they were taught, yes. They needed specific training, yes. Those came from being taught specific solo exercises and partner practice.
And at some point, they no longer needed the teacher.
Where, then, is the explanation for Modern Aikido?
Ueshiba learning from Takeda:
20 days in March.
10 days in April.
All of February.
10 days in March.
Might as well say from April to September. 5 months.
10 days spanning Mar-Apr.
Shioda learning from Ueshiba:
Fom 1941-46 was not training with Ueshiba. Supposedly spent a brief training period (month?) of training with Ueshiba in Iwama sometime after the war. Then started teaching in 1950.
Tomiki learning from Ueshiba:
In 1936 he moved to Manchuria.
Awarded 8th dan by Ueshiba in 1940.
Part of teaching staff at hombu until late 1950s.
1931- few months of training.
1932 - Trained with Ueshiba when Ueshiba travelled to Kyoto. Awarded 2 scrolls by Ueshiba.
Spent 8 years in Mongolia.
Aiki News Issue 054
Mochizuki Sensei: That same year my brother and some others had built a dojo in the center of town and I guess they were afraid that if I went back up to Tokyo I would die. Anyway, we decided that when I got out of the hospital I would start out slowly by teaching the young people in the town as I recuperated. When word of all this reached Ueshiba Sensei, he, Admiral Takeshita, General Miura, Shun'nosuke Enomoto Sensei, Yasuhiro Konishi Sensei, and others all were kind enough to come down for the dojo opening ceremony. After that, every month, when Ueshiba Sensei went go to teach at the dojo of the Omoto Kyo religion in Kyoto, he would stop in on his way there and on his way back. There were times when he would stay for two or even three days. Anyway he really liked me. Sensei would tend to stay on and not go home so at times (his son) Mr. Kisshomaru would have to come get him; that's how much he liked my place. It was about that time
that he gave me the scrolls for the menkyo kaiden (master teacher's license).
Aiki News Issue 027
From 1926 until the outbreak of World War II, O-Sensei maintained a heavy teaching schedule centering his activities in Tokyo. His
students were primarily military officers and person of high social
standing and his teaching services were in constant demand. He was obliged to travel extensively around the country and made almost yearly visits to Manchuria, then under Japanese political control.
Aiki News Issue 033
Saito: In the pre-war period he taught without explanation. Students couldn't ask questions. He only demonstrated throws.
Aiki News 047
Editor: During Ueshiba Sensei's training sessions in what way did he explain the techniques of Aikido? [1933 time frame]
Kunigoshi Sensei: No matter what it was that we asked him I think we always got the same answer. Anyway, there wasn't a soul there who could understand any of the things he said. I guess he was talking about spiritual subjects but the meaning of his words was just beyond us.
Editor: How many training sessions were there everyday back then?
Kunigoshi Sensei: There was the 6:00 am class and another morning practice at about 10:00 am. Then for people who worked in the daytime there were three other periods in the evening. Then the uchideshi could train anytime in between those hours, too.
Akazawa Sensei: No, there was nothing like that. He would say,
"O.K.", and show a technique, and that's all. He never taught in
detail by saying, "Put strength here," or, "Now push on this point."
He never used that way of teaching.
Akazawa Sensei: O-Sensei never taught exactly how to become really strong or about such things as that.