That's why Lucas has him as producer. Speilberg or Zemekis would give artistic input. Lucas doesn't want other people's input. After renting and watching the fourth disk with the documentary, I think Lucas believes if he had someone like McCallum, he would have been able to do more of what he wanted to do for A New Hope. McCallum may have no talent for making movies, but he will do the dirty work and say the dirty words to get the job that Lucas wants done. McCallum is there so Lucas doesn't have to get his hands dirty and can look like the good guy.
That's why Lucas has him as producer. Speilberg or Zemekis would give artistic input. Lucas doesn't want other people's input. After renting and watching the fourth disk with the documentary, I think Lucas believes if he had someone like McCallum, he would have been able to do more of what he wanted to do for A New Hope. McCallum may have no talent for making movies, but he will do the dirty work and say the dirty words to get the job that Lucas wants done. McCallum is there so Lucas doesn't have to get his hands dirty and can look like the good guy.