I'd like to express my views on the subject, which bleach may care about, as he/she is in a game with me right now.
I think that trustworthiness and honor are commodities, just as magical gems, troops, and gold are commodities. Both of these have some worth in that, the more a nation has at some point in the game, the more likely someone is to believe what that nation promises later on, and the more likely someone is to make an agreement with that nation. This can often be an advantage for the side in question. Consider the following contrived example:
You are at war with two nations, and you probably can't continue to fight a two front war. Because of other world-wide events, these two nations both propose a peace treaty to you. One has been shown to be untrustworthy, one has been fair and trustworthy in the past. Which are you more likely to make peace with? Chances are, you will make peace with the trustworthy opponent, as that peace means more to you. Thus, the trustworthy nation has gained.
Like all other commodities, there will be times when you absolutely cannot afford to remain faithful to an agreement, but it's good to do so while you can. That is why I gave Jotunheim a 1 turn warning prior to attacking. That is also why I gave Abyssia a 1 turn warning prior to attacking. Both of these decisions were at some cost to me, but I was hoping to show that, while my country certainly has ambitious designs, they wish to carry out those designs while remaining honorable. While I don't
expect honor in return, I hope that my actions increase the likelihood of receiving it.
Just my 2 cents.