Well, I'm doubtful that too many writers of spanking-oriented fanfiction, as opposed to the general type of it, believe that they can portray the characters in print as effectively as their original creators have--it's probably just a matter of wishing to create a spanking scene which the original author never wrote.
I'm certain that Margaret Mitchell could've written a convincing passage in which Rhett Butler spanks his eventual wife Scarlett within
Gone With the Wind, but she didn't do so--even though he did threaten to spank or whip the woman on several occasions--therefore it would have to be written by someone else (as spanking fanfic) to bring it in existence. To me, that would at least be a credible fanfic chastisement, since Rhett tanning Scarlett's behind strikes me as believable based on their characterizations in the novel.
On the other hand, Melanie Hamilton Wilkes paddling her cousin/husband Ashley (or actually anybody else either), I wouldn't find that credible, and I'd venture that good spanking-oriented fanfic should postulate punishments which are reasonably believable.
If a fanfic piece is entirely 'steeped' in the details of an original work, it runs the risk of confusing potential readers who aren't fans of it, thus I'd venture that good spanking fanfic shouldn't necessarily require an in-depth knowledge of the original source.
However, I wouldn't especially desire to encounter fanfic which is written so generically that its relationship to the original work turns out to be merely a matter of proper nouns. "That story about Kathryn Janeway spanking Seven of Nine on board the
U.S.S. Voyager, it would've been equally effective by simply substituting Teresa Lisbon and Grace van Pelt at C.B.I. headquarters into their places." No, it shouldn't function that way, I'd venture.
Thus I'd favor the 'middle ground' in the writing of spanking-oriented fanfic, so that those who are fans of the original work can appreciate its unique characters and setting, yet non-fans can still appreciate the basic account it presents.
During the fairly rare occasions on which I attempt to write fanfic featuring red-bottomed corporal correction, that's what I'm hoping to manage...

--C.K.