It's surprising what you can learn from popular fiction. Apart from containing the first full-frontal sex scenes I ever read (learnt a good deal there), Judith Krantz's Scruples also supplied me with a piece of understanding which is nothing to do with sex but which has stayed with me. Towards the end of the novel the heroine is watching her new movie-director husband edit a movie. I don't have a copy of the book these days but, as I remember, she notices how the most beautiful piece of film or exquisitely acted scene will be be cut, if it spoils the structure or pace of the move as a whole.
I can't really say that I've looked to the S&F novels of the late 70s and early 80s for much in the way of writing advice, though I'm sure the mega-sellers such as Krantz's could teach many of us a thing or two about storytelling. But perhaps sacrificing beauty is a good way to think of this question, and a better one than the often-quoted "murder your darlings", which has strong whiff of the nastiest side of your Inner Calvinist: if you love it, it must be bad, and if it's bad you must kill it. But you'll know by now, from posts like The Common Scaffold, that I don't think it's useful to beat ourselves up for a "failure" which is a natural result of how writing happens. Certainly, an alarm bell should ring if there's a bit of your story which you keep finding reasons to keep. If it were clearly wrong, you'd have cut it ages ago; if it were clearly right, you wouldn't have to keep finding reasons to keep it, because we only need to cling to things when they're threatened. But it's your own writerly horse sense which is threatening it, because it slows the pace or unbalances things; it pulls the reader out of the story and wrecks the fictional dream; or it breaks up the voice as badly as a soprano's fit of coughing.
So why do we cling? There are various possible reasons, I think. Did it cost a lot of effort in research? In writing? Do you love the non-fiction material you know or discovered (we historical fictioneers are horribly prone to this)? Does it bring in something or someone precious to you, in however disguised a form? Did it slake the deep writerly need to get something or somewhere which matters to you down on paper? Or the need to pin down an idea or an observation? Is it something someone told you is a requirement for the genre, or an agent-catching idea? Is it funny of itself, or touching? Is it just really, really well written - a piece of prose you're rightly proud of? To cut any of these out of the book would be to lose something well worth saving.
Only of course you must cut them. Your story-train may rush across Picardy at a speed even Hitler didn't dream of, or it may amble through remote mountain passes so slowly you can see every gentian, but if you don't build it right then it won't withstand the journey, and if it stalls the passengers will sooner or later get off. But that doesn't mean your darlings are worthless, or you were stupid to write them, or you're a wimp to want to keep them. Of themselves, these are all good things in a piece of writing, and as well as the fact that good writing is always worth doing, you almost certainly needed to write them before you could find out that on this train they're bad for the aerodynamics, or obscure the view of the Alps.
So next time the warning bell rings in your writerly mind don't either stop your ears to it, or let it deafen you to the piece's merits. Give yourself a chance to enjoy what was good in the writing and why it's there; let yourself know that it was good, and you wrote it. And then get out the knife.