Babak, don't you think your approach here is a bit too dogmatic. Yes, in an ideal environment every game can be played for itself, but in here this is not possible.
The way I see it, a player here carries a 'reputation' one way or another, based on past games. So, effectively, you can consider each player as a country, and each game as an individual war. Any country will act somewhat different in different wars, but there will be some similarities and common trends.
So, if you have the image of North Korea, chances are any game (war) you take part in will be a tough experience. While, if your foreign policy is Swiss-like, chances are people will be willing to give you their money (support).
Game-long alliances are part of one's reputation in this game, and actually it's the same in real life (USA-Israel, ENG-POR to name a few). As long as you don't take it to the extreme, and don't do it with the same people across games, it is ok.
Additionally, about PPSC, there are also some benefits from this variant, and there are some negatives to WTA also. Not everything is black and white. Unless you're role-playing the Vatican :)