I was listening to the Writing Excuses podcast not too long ago when they had a guest on. I wish I could remember her name, but I'm lazy and I don't want to look it up. The subject was on fleshing out secondary characters and when they should have back stories and when they can just be spear carriers (a term stolen from theater). Her answer, which isn't for everyone, was that anyone that gets a name will get a story. She said "no one gets a free ride" and her characters have to pull their weight.
So I wondered about my own use of secondary characters. While I don't think stretch a one-time secondary character out into their own story, I think I really do tend to turn characters that could be a one time thing into deeper characters. I guess the way I think about it is this: if I flesh out a secondary character and give them a good backstory and motivation, it can only help the story overall. I don't have to detail all those motivations and background elements within the text, but that information can certainly inform my decisions for how they act.
And I'll even admit that I've turned characters that I created out of necessity for a scene into major players as my stories have matured. I might think, "hey, I need a character to fill those role for just one scene," but then I end up creating something I like and finding more uses for them as the story goes on.
At the same time, I think you have to be careful. It can be a black hole when you get started. Creating characters is fun and the more you do it, the better you get at it. You have to be careful that you don't start creating all kinds of secondary characters that aren't that important, or worse yet, that overshadow your main character. And when your side characters are more interesting than your protagonist, well, that same woman from Writing Excuses had only one solution: kill 'em off.
Keep your eyes open for my debut novel, The Paladin.