There's an old saying: better to keep your mouth shut and be thought a fool than open it and remove all doubt. I would think that several people in this thread might benefit from remembering that...
As to the act itself, well I wouldn't have done it, for reasons that have been well explained by Smilodon. I've been in alliances that have done such a thing - most recently in Olynthus, where the action really was for the best for the members of the alliance disbanded IMHO - but in general, I prefer to play with alliances that DO have a sense of honour about the game, and it's amazing how successful they can be. Find like-minded pact partners, and the combined strength is awesome; far better than waiting for the knife in your back. Do something like this and it's certainly true that your chances of being trusted are significantly reduced; it's a matter of judgement as to if that's worth it or not.