A good editor is invaluable when starting out. Later on it perhaps depends on how complex the story or writing is. Editors can be looking for similarity not originality. The final script a poor representation of the original story or article. An article of mine, which was picked up by Wikipedia for its originality and informative ideas was given forty edits from around the world! It was like a mechanical invention rather than an interesting article in the end!
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.