Scott1 the implications of the city as accepted as a ficitonal account and non-fiction have two bearings on the theism debate in large:
1) As non-fiction the account is proof that God preforms mircles and divine interaction. If theists could do two things
1) discover the biblical cities of sodom and gomorrah
2) assess with strong validity that the act of sulphur raining down was a divine act (by either finding no volcano's near it or volcano not bury it in ash) a strong arguement for God as dipicted by the Jews, Christians Muslims and Bahi would exist
As non-fiction it could be used to positon why "bad things happen to cities" Many a christians like the baptist pastor F Phelps have used the story of sodom and gomorrah to explain the tsumanis that hit shri lanka and thailand the the destruction of new orleans.
The non fiction becomes a thing of pointless debate however evidence of god destroying a city for sinning would shed a new light on "natural distators" of today.
As far as your question "how much does it myth or fariytale " not a whole lot. I find it interesting as a topic of debate because of the similarity of the dimise of pompeii to the biblical cities and the contrast on both
1) how information is assessed as real for both
2) the direction of archologists vs theologican researchers in terms of gathering evidence.
In short as fiction it is an allegory and thus subjective, as non-fiction it would be more objectivve in interpretation which holds the most signifigance to me from a metaphysical point of view.
Last footnote, the cities have not been found. Read the links everyone has pitched in, including the ones I put in the opening post. Quite a group of interesting reads.