Why some aikidokas don't like to practice with beginners?
Because when you practice with a (very big) new beginner, then you know what is your real level in aikido not the level you think you are. A (big or not) new one does not know where to go since he does not know the technique, you have to lead him. Sure it is easy to break his face or something if he does not do what you want but to my taste it very low level aikido.
A beginner is a perfect test for your aikido. After the practice with him or her, you know what you have to work on to better yourself.
That's good in theory, but in order to lead them, they'd have to be totally compliant, right? So how is that "a perfect test for your aikido"? And if they're not totally compliant, and they don't know how to take ukemi, how do you practice your techniques -- really practice them, not just wave your hands -- without injuring them?