Lanefan
Victoria Rules
The players may or may not be co-operative among themselves, that's up to them; but in any case those players are always collectively competing against the game and its associated in-setting challenges. The DM is the referee over that competition; and that competition IMO should be much more war than sport.I don't think that I've seen roleplaying as a refereed sport since... well, since the 90s. My view on RPGs is that they're a collaborative not a competitive enterprise. A referee is needed when players are competing against each other - a neutral party is there to make sure that things are fair for each player/team and all of them are following the rules.
In an RPG the players are cooperating with each other, not competing. So where does the need for an impartial judge come in? If you're looking at it as a players vs. GM situation then the players have already lost - the GM in any game can crush the players mercilessly if it's a player vs. GM situation because there is no referee between the GM and the players.
If it arises that the players start competing in-character among themselves the DM also has to referee that.
The tricky bit is that the DM both presents/assigns the challenges and then has to referee the game of players-vs.-those-challenges. This is why I'm very much in favour of challenges, adventures, etc. being set well ahead of time without knowledge* of which specific PCs will interact with them, such that when those interactions occur the DM can act as a neutral referee.I see the role of the GM as a fellow player, not a referee or a competitor. The GM presents the world in the moment and the players play out their reactions to it and the GM then reacts to that. That cycle produces a collaborative game. Everyone has bought into the idea that we're going to play a game of pretend where our dice rolls have meanings, so the need to enforce rules or look for cheaters should be at a minimum - anyone who "rejects the premise" of the game and starts to find ways to "cheat" tends to IME get called out by their fellow players before the GM even needs to get involved (cheating in an RPG is like cheating in solitaire - you can do it but why?)
* - beyond only the most general info such as average level of the party, that sort of thing.