Here is another point for those in karate (the striking arts) who also train in any of the grappling arts.
There is a belief that Ueshiba's aikido did not actually develop from aikijitsu. Instead, Ueshiba learned Pa-kua Chang (eight trigram palm) while living in China. Much of the circular stepping, spinning, and throwing are commonplace in the solo hand techniques (as well as the partner drills) used in Pa-kua.
As a Pa-kua man, I see little difference in the movements of jujitsu and those practiced in Pa-Kua's circular (as well as linear) movements. I came to this conclusion mainly due to the obvious fact that human anatomy is universal; that a punch in the nose is still a punch in the nose, regardless of how it's executed. Also, that muscle memory is improved due to constant repetition: i.e. both solo and partner training.
For example, an outside wristlock and throw in jujitsu & aikido is identical to one of the two main divisions within Pa-kua: single palm change (180' degree turn). The other being a double palm change (360' degree turn).
I'm wondering if any of you have ever tried to practice some of your jujitsu/chin na techniques solo instead of with a training partner.