Rules for Social Interaction
Role-playing games use dice to resolve events and simulate results that are out of the control of the intentions of the character. When a character wants to say something, they can; when they want to pick up a stone and hurl it, they can. What they cannot control is whether the stone hits what they want -- or whether the words have the impact that they want. It the RPG world that is resolved by the dice.
The problem is that these situations are not similar. It is difficult to have a unified set of rules that take a consistent approach to both social and physical combat while allowing the flavour of each contest to be preserved.
Social combat is :
talking your way passed a guard
charming a barmaid
swaying a crowd
convincing a judge of your innocence
changing someones opinion
One can argue that social combat should have no rules attached. You could make the same argument for physical combat. The argument goes: Let the description of the players merge with the combined will of the group, the inclination of the referee, and the impact on the story. In this way you have preserved limitless freedom of the players. With one big caveat... freedom to succeed of fail at the discretion of the referee. All players know that any success they have is based on the disposition of the referee. They can reward bad playing as well as good playing. They define what is bad playing and good playing. They can have any agenda they choose.
To most players, that level of open-ended play is not satisfactory. They can enjoy it for a while, but after a while they give up. They bend and warp to play to the referee, not the world. The narrative story loses its importance and the sub-textual cues of the GM become all-important. A great GM, immune from any impartiality, able to discern and adjudicate perfectly, may be able to avoid the inevitable crash of a game-without-rules, but most will fail. Many fail quickly and miserably.
A game without rules must fail because it is not a game without rule -- It is a game with one rule. The single eye of Sauron (the GM) adjudicates all things. One rule to ring them all.
A game that is all rule and no heart is similarly doomed. The clatter of dice and off-topic conversation dominates this game. At this point we may as well take up Yatzee.