I don't ever read fanfic about a show I've never watched. I don't see a point in it when I don't know the characters. I have read the odd MFU crossover (and enjoyed it) but it didn't make me want to watch the other show. The most that I did was look for photographs of the characters from the other show, found them unattractive and that was that.

However, I wouldn't regard it as original fic. Fanfic is usually strongly focussed on two (slash) or very few (gen) characters from a show. Original characters do appear but they usually only play a supporting role (unless they're Mary Sues). In fanfic I wouldn't have it any other way. In original fic I expect a little more.

Obviously fanfic as opposed to original fic has a certain setup and character description that appeals to fanfic readers even if they don't know the show. I don't really understand it but there are numerous examples for this, not least in the comments to your post here.