Richard. I said it doesn't necessarily mean, you say it undoubtedly does.
O.K. What if the two arts are incompatible?
Secondly, did the whole of Judo lead to what you say or certain parts? Did the whole of a striking art lead to improved ateme or a specific part.
Add to that which parts of that 'other' training did you find you automatically did in Aikido which in fact you shouldn't of. In other words it got in the way 'by accident'
What I'm taking from your comments is you think that another art can get in the way of Aikido or people may not be capable of applying skills gained in other arts to Aikido without damaging it. Was that a bad inference on my part?
Out of curiousity, what art would you say is incompatible with Aikido? Would it be one based on starkly different principles? Maybe I'm naive to think that any art would help develop skills (even if they are just body skills) that would improve someone's Aikido.
Obviously, there are arts that compliment Aikido like Judo, Daito-Ryu, and Iaido, which utilize similar principles. However, I would argue training in a very different system, like western boxing or wrestling, would result in improvements in one's Aikido.
The main argument against this idea seems to be the possibility of one becoming confused and trying to apply incompatible principles from one art to another. However, I tend to believe that people are capable of discerning what works and what doesn't. There will be stumbling points, but that's all a part of learning.
With similar arts, like Judo and Jujutsu, I think you can directly apply principles and techniques. I study Hakko-Ryu and certain Jujutsu techniques were second nature to me because I had learned something similar in Judo. Taking Shotokan Karate as a youth was beneficial to Jujutsu related atemi.
With dissimilar arts, like the boxing or wrestling previously mentioned, there may be principle incompatibility and you may not be able to incorporate specific techniques into Aikido without altering to the point it is no longer Aikido. However, even if those principles and techniques are not brought in, the body skills gained from those arts would greatly benefit an Aikidoka. The way boxers are wrestlers train, an Aikidoka is bound to gain improvements in hand and foot speed, explosiveness, and strength.
The biggest issue I see in regard to cross-training in other arts is not dedicating enough time to either art. Unless you are training frequently, it may be questionable to expect to excel at both arts.
One of the biggest benefits I can see in cross-training in another art is an element which I've always thought is missing in many styles of Aikido as well as Hakko-Ryu, sparring. I think it's hard to argue that the mental benefits gained from being in a pressure situation would have a positive effect on one's Aikido. It also gives on an opportunity to try to apply certain Aikido/Jujutsu techniques on a highly resisting opponent in a pressure situation. I've had the opportunity to roll with a judoka and a few BJJ practitioners a few times recently, and it was enlightening to say the least.