I do a lot of stories off the cuff. One thousand and thirty, before I get started on today's. Some, like We're Mostly Harmless, I Swear, wind up becoming the cornerstones for novels.
But I'm never lazy about it.
I could have easily copy/pasted my Instants about Amity into the novel-that-is and few would have been the wiser.
I'm not like that. I can't let myself do that. It's a form of cheating and - at least in my humble opinion - it prompts the reader to skip over vast passages of content until they get to something 'worth' reading.
I experienced it myself when reading Jean M. Auel's Earth's Children series. I loved the hell out of Clan of the Cave Bear and I was eager to read the next one in line. Until I realised that every Jondalar chapter was [in my humble opinion only] badly-written male power fantasy sex scenes with little to do with the plot. I literally skipped half the book until sex was not in the equation. And in later books, I skipped the travelogue sections where I thought the author was obviously padding. As well as the unnecessary sex scenes.
It's a rule of mine. Never make reading boring for your audience. If you do that, you lose your audience.
I could probably write an Ace romance novel. It won't sell, because most people are in the romance section for the smut. But I could do it nonetheless.
Anyway. Back to "cheating". There's various degrees of doing that sort of thing. If I can't work out where I'm going in my current plot point, I describe something. That's allowed, because Speculative Fiction has lots of room for that.
What I don't do is randomly insert an Instant into a novel.
1) It encourages loss of audience by breaking them from the narrative
2) Stories are capable of change without warning and deserve a 'pure' rewrite to get them done properly [Just look at the First Contact scenario in We're Mostly Harmless, I Swear versus how it happened in The Amity Incident. Vastly different because I thought about the science]
3) I want to be as involved in my writing as my audience is in the reading
4) It's really, really lazy and I don't want to do that.
Yesterday's Instant coincided with a piece I was planning to write, later that day. As in, the bit I put into my novel has the exact dialogue I wrote, yesterday. But I still rewrote it so that it would fit into the coherent whole. I figure I can get away with the dialogue because there wasn't heaps of it, and a necessary change of perspective makes a world of difference to it.
But that's my opinion.
It's my readers' opinion that matters. The more readers I have, the more money I make. The more I can write to gain more readers, blablabla...
As a writer striving for financial independence, audience is literally everything.