Creature: (irritated) I cannot teach him. The boy has no patience.Luke's head turns in the direction the creature faces. But there is no one there. The boy is bewildered, but it gradually dawns on him that the little creature is Yoda, the Jedi Master, and that he is speaking with Ben.
Star Wars - The Annotated Screenplays, page 175
This scene is completely ruined if Yoda is in the prequels. Any kid watching the prequels before Empire Strikes Back are robbed of the experience of discovering that this is Yoda together with Luke. I don't know at the moment whether or not George Lucas always considered to have Yoda in the prequels. He certainly weren't in the backstory of A New Hope, as Lucas didn't invent him before the work on the script of Empire Strikes Back began. When he needed a new teacher for Luke after he decided to kill off Ben at the end of A New Hope.
Yoda do of course not need to be in the prequels. The story of Anakin's transformation can be told without the future Darth Vader knowing who the teacher of Obi-Wan is and where he lives.
I suspected it is the popularity of Yoda that is the reason for his inclusion in the prequels. But my feeling is that the way he is portrayed in the prequels is retracting from the character. He is less of a Jedi Master with his action in the prequels. It is a clear example of less is more.
The most in your face example of that is the lightsaber duels of Attack of the Clones and Revenge of the Sith. It is out of character in two senses. First of, making Yoda trow away his cane and then jumping up and down like a yo-yo on speed is just ridiculous. Secondly and more seriously, if Yoda is the most powerful Jedi Master he should be beyond fighting with a lightsaber. True mastery whould be to have him deny others to fight him. It is mind over body.
Lawrence Kasdan: "I'm a big Samurai movie fan, as is George. The stories I find most interesting are stories of Zen education and the Zen master teaching a pupil how to transcend physical prowess into some kind of mental prowess. That's what all the training sequences are about. My favorite director is Akira Kurosawa, and Star Wars was inspired by his film The Hidden Fortress, so George and I had an immediate connection there. All through Kurosawa's movies you have the idea that it's one thing to be physically adapt and something else to be spiritually adept."
Star Wars - The Annotated Screenplays, page 180
I think Lucas must have forgotten all the lessons he must have taken from Kurosawa as inspiration for the Saga. Because the Jedi knights of the prequels are pale shadows of what they could have been.
Another lesson forgotten is that Yoda has to be believable. All involved with Empire Strikes Back knew that if Yoda looked like a muppet it would not have worked. Look at the picture above. Clearly they succeeded. Now look at the picture below. It is still the acting and voice of Frank Oz, but this is a soulless muppet. The charactermodel is just terrible. It simply is not Yoda.