I've probably used pre-gens more than any other author, with the possible exception of Harri Polsa.
Pre-gens, as mentioned, are most useful to an author when the story's background and pre-existing relationships really require one. All the SHUA mods, thus far, have used pre-gen parties (Doc Sabre's pre-SHUA superhero mod, Super Defenders not counting). Steve Gustafson's The Adventures of Wonder Woman allowed players to create a character to adventure with his pre-made Wonder Woman, or use a second pre-gen, instead. Someday, I hope to do a big, make-your-own-superheroes mod, but that's still far down the road.
Pre-gens are also useful for mini-mods, where authors are spared the trouble of character creating when that labor might take longer than actually playing the mod.
If a mod has well-written pre-gens, like Paddi Whisperleaf or Millar Jade or the Teenage Mutant Ninja Turtles or the X-Men, I doubt many players will mind the imposition.