The primary reason is that, by nature of the game, the party is required to accept new PCs into the party and stay together as a party, even if they might not do so in real life. It is not practical for a 4-PC party to split up and each essentially set out on his own solo campaign because, for whatever reason, each player decided that it makes sense for his PC to go off alone. It may be realistic from the perspective of the PC but it is impractical from the perspective of playing with a gaming group. Similarly, new PCs join the party somewhat artificially. It is difficult (and dull) to spend a lot of sessions doing background checks, building a rapport, establishing trust and finally accepting a new PC as trustworthy. For expediency, the new PC gets inserted in the group and is assumed to be trustworthy by the group, simply to avoid bogging down the game.
For example, in the game in the article, did the party really have a choice of accepting the Sunaj-turned-slave? I'd say no. They couldn't really say, "Sorry, we have no reason to trust this new PC so we don't let him in the party. We get rid of him or leave him behind." If a PC is a traitor, the GM must be very careful that he compensates for the artificiality needed to keep the game from bogging down. Forcing the party to accept a new PC for meta-game concerns and then making that PC a traitor is unfair without some counter-balance. And, it is very difficult to provide this counter-balance. Possible but difficult. The campaign should have several mechanisms where the traitor might be revealed prematurely or hints that would lead the party to suspect one of their own.
A secondary reason is that the betraying player often has much more fun than the betrayed ones. It is usually effortless for a player to leverage his out-of-game player friendships into in-game PC trust. Enjoying playing a traitor is often not so much a gaming challenge as it is enjoying the manipulation and exercise of power over other players (i.e. a power trip). Another reason is that, once a few betrayals happen, future games may be filled tedious research and paranoia as the PCs search for traitors which aren't there before beginning the actual adventure. Yet another reason is that, as the result of a betrayal or two, the players may not interact much with NPCs, besides killing them, and, in my world, I put great effort into building PC-NPC family relationships, friendships and alliances. And, finally, betrayals are easily misplayed: the GM may misappraise his players' maturity and emotions; the plot may spoil the game and even the group.
I'm not saying that it cannot be done and cannot be enjoyed. I personally dislike games with traitors because I prefer games with external, us-against-the world goals and not internal, party-strife ones. But others may feel differently.
I take a dim view of games with traitors.
The primary reason is that, by nature of the game, the party is required to accept new PCs into the party and stay together as a party, even if they might not do so in real life. It is not practical for a 4-PC party to split up and each essentially set out on his own solo campaign because, for whatever reason, each player decided that it makes sense for his PC to go off alone. It may be realistic from the perspective of the PC but it is impractical from the perspective of playing with a gaming group. Similarly, new PCs join the party somewhat artificially. It is difficult (and dull) to spend a lot of sessions doing background checks, building a rapport, establishing trust and finally accepting a new PC as trustworthy. For expediency, the new PC gets inserted in the group and is assumed to be trustworthy by the group, simply to avoid bogging down the game.
For example, in the game in the article, did the party really have a choice of accepting the Sunaj-turned-slave? I'd say no. They couldn't really say, "Sorry, we have no reason to trust this new PC so we don't let him in the party. We get rid of him or leave him behind." If a PC is a traitor, the GM must be very careful that he compensates for the artificiality needed to keep the game from bogging down. Forcing the party to accept a new PC for meta-game concerns and then making that PC a traitor is unfair without some counter-balance. And, it is very difficult to provide this counter-balance. Possible but difficult. The campaign should have several mechanisms where the traitor might be revealed prematurely or hints that would lead the party to suspect one of their own.
A secondary reason is that the betraying player often has much more fun than the betrayed ones. It is usually effortless for a player to leverage his out-of-game player friendships into in-game PC trust. Enjoying playing a traitor is often not so much a gaming challenge as it is enjoying the manipulation and exercise of power over other players (i.e. a power trip). Another reason is that, once a few betrayals happen, future games may be filled tedious research and paranoia as the PCs search for traitors which aren't there before beginning the actual adventure. Yet another reason is that, as the result of a betrayal or two, the players may not interact much with NPCs, besides killing them, and, in my world, I put great effort into building PC-NPC family relationships, friendships and alliances. And, finally, betrayals are easily misplayed: the GM may misappraise his players' maturity and emotions; the plot may spoil the game and even the group.
I'm not saying that it cannot be done and cannot be enjoyed. I personally dislike games with traitors because I prefer games with external, us-against-the world goals and not internal, party-strife ones. But others may feel differently.