Maybe I don't like training with beginners because after four years of Aikido practice I start to believe that my technics may work sometimes. But with beginners (it's so frustrating) they never do. I always discover the weak points of my Aikido with them. It's true, you can learn very much from beginners.
This is definitely true. Trying to apply a technique on a beginner is actually a more realistic situation. If you were attacked on the street, chances are your attacker won't know how to take the ukemi from whatever technique you do.