After considering the surface of the question, then the actual results of those who begin to achieve the higher level of aikido, I would have to add this small point ....
You need to get out and practice other martial arts besides Aikido to achieve a high level of understanding for Aikido, as well as practical proficientcy beyond classroom practice.
I hate to beat the subject to death, but just as the classroom is the beginning of knowledge, you must eventually leave the classroom to go outside.
For those pessimists with the computer age recognition, you still need out side people to bring you things if you are inside, and they do connect you to the outside world as your emmissarys.
I guess we could depend on this type of mentality of waiting for things to be brought to us as we practice, but then ... each of us has our own rate of developement to consider, and sometimes waiting in a room in not enough.
So, as much as I agree that teaching aikido is necessary to gain a higher level of proficientcy, it is not always the simple teaching of other students, sometimes it is verifiying and adding to what you know by examining what the rest of the world has to offer.