Actually, I think Kurtz' influence on the films is highly, highly overrated. He was terrible at his job and ruined his career through his inability to do it right, which is why he hardly ever worked after the mid-80's. On the Star Wars films he nearly ruined Lucas financially and physically, and although its obvious the films were stronger with Kurtz around, Lucas had every right to let him go.

American Graffitti was mostly produced by Coppola and was such a small film that it would be hard to have any kind of catastrophe. Star Wars' budget ballooned quickly, and was fortunate that the American Dollar was worth more than the pound at the time it was filmed, otherwise it would have been a fiasco, and the film was so badly scheduled that Lucas was shooting up to three scenes at once at the very end; the stress made Lucas admit himself into the hospital! I know some elements are out of any producers hands but when the director nearly has a heart attack from filming a low-key medium-budget film, something is seriously wrong.
Flash forward to Empire--Lucas finances the film himself, and puts all of his personal fortunes into the film. The budget is set at 15 million. The show quickly gets behind schedule and the budget increases week by week. Kirshner takes way too long to film scenes and Kurtz, the producer and the man responsible for moving the show along efficently and smoothly, lets the budget DOUBLE to 32 million, all the while getting more behind schedule. A bank loan has to be taken out by Lucas to cover the film, which also goes over budget. All of Lucas' personal funds are invested in this disaster of a production, and the finger is pointed squarely at Kurtz. The film was a success, luckily, and Lucas made his money back and thensome, but for all the grief and incompetance Kutz caused, it is no wonder they never worked together again.

THEN, Kurtz goes on to direct The Dark Crystal, another flop. He returns to producing, and makes Lucas' friend Walter Murch's directorial debut Return to Oz--which, surprise, surprise ALSO goes overbudget! Lucas has to bail out Murch himself, thanks to Kurtz' incompetance. It would be one of the last time Kurtz ever worked.

So was he a good producer? Abosolutely not. Although he obviously cared about the content more than, say, McCallum, he was negligent and incompetant in his role, and despite the fact that he lached on to Lucas when Lucas was still young and creative, Kurtz' status as "Savior of the OT" is a complete fabrication by the meer coincidence that ANH and ESB are better than ROTJ. McCallum is a million times the producer Kurtz was, though unfortunately associated with poor material, the opposite of Kurtz--but the fact is that the man did his job. There were no scheduling or budgetary issues on any of the PT, even when the script for AOTC was completed until the week before filming. Despite the fact that Lucas may have lost his touch, if i were making a film i would definitly want McCallum and not Kurtz with me!