Shouldn't treaties be binding, or at the very least a heavy penalty imposed for breaking them ? And if the AI can break treaties, why can't you ?
I think this really opens up a VERY insteresting debate as to how diplomancy could operate succesfully.
I've always considered the fact that you are 'locked' into an agreement to be very frustrating. However at the same time, I most certainly never wanted to be on the receiving end of a deal break either.
If, depending on the type of treaty/pact/alliance broken, and also depending on the time remaining to said treaty/pact/alliance, the AI or player were to receive a severe penalty in some form of monitary fee or diplomatic disposition with other factions or perhaps even unrest within your own borders. There would be definate incentive to keeping your word (and feeling safe that the AI would do the same) yet have the ... oppertunity ... to take advantage of situations that would otherwise be unavailable to you. Likewise, the AI would then be given the ability to sometimes make your day a living @#%#. 