Okay, I think line-editing is great. Lines should be as information-dense, melodic, and insightful as possible, and you can't get there without a lot of work. However, I don't think a line edit is rarely the difference between selling and not selling, or pleasing and not pleasing your writer.
Too often I see people get a bunch of comments back on their manuscript, whether from editors or agents or other readers, and all they do in their revision is they shift the words around a little bit. For instance maybe the editor says, "I didn't know why the character was doing this thing?" so the writer puts in a line where they explain why they're doing that thing. Or people were like "Huh, this motivation felt weak" so they put in a few lines where the character is like, "I did this thing because I am angry, and I am angry because the tyrant killed my people."
And at the end of the revision, you've still got the same events in the same order. Which, to my mind, means you've fundamentally got the same book.
I'm not saying that the words on the page don't matter, but…wait a second, that's exactly what I'm saying.
When a person reads a book, they're not experiencing words on a page. No, they're experiencing a "living dream" (as John Gardner put it) that is conjured up by the words on the page. Now of course the tone and texture of that living dream are affected by the words on the page, but when you revise a book you need to be very clear about what you're doing. You're not trying to alter words, you're trying to alter the experience of the person who reads those words.
If viewed this way, individual lines and paragraphs become much less important. You don't remember individual lines (except the most beautiful and insightful) of a book. Still less do you remember the details of their thoughts and backstory and motivation. What you remember more is the voice, the tone, and the events. Voice and tone are, to my mind, a complicated thing to change, but I do think they're more dependent upon the events in a story than anybody might like to admit. Your work has a certain voice, but the deeper layers of the voice only come out when your characters are placed in fresh and surprising situations. Tone, too, is the result of expectations, set at the beginning of the book, that are then either undermined or reinforced by subsequent events.
Which is all to say: I strongly believe that a story is composed of things that happen.
They don't need to be huge things. I think a conversation is an event. In fact (at least in my stories) most events are nothing more than conversations. But the things that happen in those conversations matter: they are the result of characters with differing goals who want different things from each other.
And when I get feedback on my books, I don't go in and tinker with the dialogue or the descriptions, I go and think about those conversations: I delete scenes, move them around, and add new scenes. I alter characters' motivations, which results in scrapping some scenes and rewriting others. And when you read one of my revisions, you often know exactly what I did: "Oh my god, in this draft he accepts her proposal instead of refusing it!" or "In this draft her parents threaten to ship her off to reform school!"
I am not tied to any specific outline or plot. Nor am I even tied to any particular story. What I am tied to, if anything, is my search for the spirit of the story: the thing that compelled me to write it in the first place. Oftentimes, finding that spirit means scrapping almost everything I've already written.
In the case of my current book (which I'm exceedingly pleased with, as you may be able to tell from the tone of this post), the plot has changed drastically at least seven times. And each time meant revising the book to add new events and take out old ones.
If this is true in a book that contains no violence, no adventure, and consists mostly of conversations (it's a love story, more or less) then think how much more true it ought to be for plot-driven works: for fantasy or science fiction or thrillers.