I feel that what occurs in the dojo is just practise for the real thing. Its how you use it and go with the situation in the streets-I think jiyu waza was created for this purpose. The sad truth is the more experience with "real situations" you have, the better you get in your MA be it aikido or otherwise. (Unless we're talking oyama karate, muay thai and JKD...hee )
I would disagree with you as far as my experiences in aikido dojo. It does not do a good job of preparing you for a "real fight". I don't believe jiyu waza was created for this purpose either. Aikido is a DO art and as such it is designed to teach you principles. Those principle can be transferred into techniques that can be used in a real fight, but it is not the same thing as "preparing you for a real fight".
I won't assume to know really why jiyu waza was created at my experience level, but I will say from my experiences, that it does not prepare you for a fight. For me, it helped me transfer the principles and habits i learned into instincts. That certainly is a step closer to being prepared for a real fight, but far from why we focus on it, IMHO.
You are correct, no one will attack perfectly. As such no one will defend perfectly either. There are things you must learn or do to compensate for the "imperfect" situation you end up in.
I would also disagree with the statement "the more experience with "real situations" you have, the better you get in your MA". I have found the inverse to be true, at least in regards to Aikido.
In "real situations" as you stated, you are not perfect in your attack. Therefore, you cannot learn good technique and it is difficult to learn the principles. (at least as far as aikido is concerned). "realistic training" can amplify your bad habits, and reinforce them.
Can we train more realistically. yes there are ways to do that, but even they are done in a controlled matter. However, I believe when you enter this realm, you will find that you are doing something other than aikido and are missing the point of why aikido (a DO or WAY Art) was created.
The principles of aikido are universal in nature, but aikido as a methodology does not make for a good way to train for street fighitng or self defense if that is your cup of tea, and training for this does not make for good aikido.