More than half of all my the fanfics archived on Fanfiction.net are one shots, ranging from drama pieces to just smut. Most of them were conceived as "spur-of-the-moment" and remain as such--because they're meant to be that way.
On that note, you can never avoid requests from people that ask you to continue the story, even if it was just meant to be a one shot. Yeah, it's a compliment, meaning they like your story and want you to continue. I would sound ungrateful saying that sometimes those requests get pesky from time to time....and sometimes it's like they "get mad" at you for not continuing the story.
I write a lot of one shots because I get so many ideas. I envision a detailed scene that isn't necessarily tied to any factors that would make it a full length fic. Sometimes, I don't even see the potential or necessity to make it a full length story because I imagine it as great the way it is, and writing more would just destroy it. It's like a work of art--if you work it too much, you only destroy it further.
I go with my instincts--when that voice says STOP, I stop. I do what my muse tells me to do--I edit, edit and re-edit until I'm completely satisfied with it. I don't only write for people that like my stories, I also write for myself.
My new smutty Shinji x Orihime, "High" was meant to be a one part one shot but decided to give Shinji is cake. It will be a three part one shot, just like my old Itachi x Sakura, "Backfire". I've chosen not to complicate things (involve Ichigo) because that would just ruin the point that I wanted to get across--that Shinji has the hots for Orihime. Sorry, but no more after that :)
But I'm not done playing with Shinji and Orihime yet.