I think there is some macho idea that you can learn just by seeing. I don't believe that at all. You can learn techniques and replicate what somebody is doing, but if we are doing aikido properly each technique is unique (because we are responding to the movement of uke, which varies).
Underlying aikido are the principles - these need explanaition. You may be able to follow principles with some physical explaition (e.g. gesturing etc when you are actally training) but I think it may take much longer.
The same happened in my swimming - I read one chapter of a book on swimming efficiency and within weeks I was swimming far far faster. I could swim before, but my speed was purely through fitness rather than technique. I had seen others in the club swimming fast, but now when I look at them I understand WHY they swim fast.
I've never found a good book that really explains aikido principles concisely and well (without getting obscure). Maybe the aikido community should get together with some sports physiologists and psychologists and write an excellent scientific book on aikido principles that would benefit everyone in a direct and practical manner.