Interesting. All I can say is that I've never played in a Mafia game without a cop (or that wasn't under the alternative setup I described). Kind of assumed that to be the standard role everyone expects - Mafia, a cop, possibly some other craziness for Internet games. (I should warn you that when I last played Internet Mafia, it was 2004, and had rules like "you can't switch your vote after you make it.") I certainly agree that there's no reason why you can't have a game without a cop and still have it be balanced.
EvilTom, re more story style games: Yes, if the clues are too blatant, then a perceptive townie can connect the dots, post their logic, and watch as a hail of votes strikes the target. It's not necessarily biased against scum though. For a similar experience, you can buy a variety of "host a murder mystery" games often based on the same premise - slowly hand out clues through the characters, try and figure out who the murderer is by the end. Both of the ones I've played of these had the answer at the end be about as clear as mud, even if we'd theoretically broken character and just shared our clues / dark secrets. In cases like those, where the clues are too obscure, it'd have been biased pro-scum.
I'd say the bigger problem with story style games is... well, you want to have everyone on the same playing field. But that can be tricky. Themes help, but then there are issues of how hardcore people are. Make the clues too easy and the person who's a complete Final Fantasy nut solves the game early, but make them incredibly subtle and the more casual fan is just kind of along for the ride assessing others' arguments. Well, and watching for the normal Mafia tells, like the scum teaming up on someone who's on the right track.
Since I was riffling through the archives today, the closest thing to that it seems the DL has done was Pretty Princess Mafia, which seemed more like a roleplaying exercise dressed up in Mafia rules. Which is cool! Roleplaying is fun too, though I'd argue that a forum isn't quite the best avenue for it.