He was a producer. It would be different if he was the one who brought Lucas in as a hired hand to produce this set of films he created and financially backed. He didn't. He's there to make sure Lucas has everything he needs to create his vision. That's all a producer is there for.
With Episode V, the budget, under Kurtz's watch, got out of hand and Lucas let him go because of it. Budgets go out of hand all the time in films and sometimes they do in fact make the difference. That doesn't change the fact that Kurtz went over budget. You've read the stories about how "cheap" Lucas when it comes to his money in projects. He's not going to spend a cent more than what's neccessary for a film.