I don't think he belongs in A New Hope at all. You've got a fine space adventure with smugglers and wide-eyed farmboys and an evil Empire, then out of nowhere you're in the middle of a swamp with a green elf for ten seconds, and you never see him again. Even people who did see the prequels, and did like them, would be wondering what in the world was the purpose of that shot. Is Yoda going to go to the Death Star and duel Darth Vader? Is he going to lead a Rebel army against the Empire to avenge Alderaan? ("To FOB Yavin take me!") Is he going to reunite with the enigmatic Chewbacca and ride around on his shoulders again? Will he show up during the Battle of Yavin and pull off a stunning rescue with a cloned wing of starfighter pilots?

Nope, he'll disappear for two or three hours (or years, in movie time). Then he'll pretend to be an idiot with a flashlight fetish. There's no reason for him to be in all the movies; Yoda, like most characters, has his entrances and his exits, and they are used to dramatic effect. They give the story an ebb and flow. This push among revisionists (principally Lucas, but also several commentators even on this site) to have all the characters static, unchanging, and always present would take away that ebb and flow, and make the series flat and less interesting.