I'd like to add a few comments to the table.
First, the game is primarily about getting to 18, but not always. I don't mind a gam-long alliance, but such alliances are dictated by the board, not by me deciding it as such. In each game, players should always weigh their options. If a stab is beneficial, take it. If it's not, don't. The game is entirely about creating and taking opportunities. If an opportunity to stab doesn't arise, I'm not going to stab. If the opportunity does come up, I may stab. I may not. Timing is everything.
Therefore, if you're seeing game-long alliances, either they are that way out of necessity, ignorance, or the fact that someone is new to the game.
Second, michigan man stated that it's better to remain in a game long alliance than to face a kamikaze player the rest of the game. To this, I have to say that if someone is willing to kill themselves getting back at you then either (a) the player is new or (b) your diplomacy has failed.
It can be a good tactic to throw yourself at the stabber if it means you can get them to change. However, I've been in many situations where I initiated a stab on someone and later had to convince them that I would back off so that we could work together again.
In other scenarios, that same guy would get stabbed by a different player after I stabbed them and I turned around to work with them so that I could benefit more greatly from their units--just to stab them a second time or not at all.
I believe it's better to take risks. You win some and lose some, but can't win if you can't put up a risk to try.