I've played a couple games with binding diplo agreements. Not my cup of tea. I've played Diplomacy the board-game as PBM (and later PBeM) since early 80's, and not being able to backstabb if you really really truly need to is just weird. Everybody builds up their own reputation. I wouldn't count on anybodys word in game as long as there is no clear advantage of sticking to his word (like he is getting something useful from a war, like new land or just being able to hold down an expanding neighbour), and usually not even when it would be of great benefit to stick to their word. It is, after all, just a game, and sometimes an un-timely backstabb can make the game more interesting (especially if there is a good story to it
).