I was at peace with all the nations, and had a mutual protection agreement with France. And ofcourse they went to war and dragged me along. So I shipped out whatever useless war units I had. And as soon as I made my way to the enemies boarder the French took a u-turn and stole one of my cities. Then totally invaded and began stealing...What the hell? Why did they do that? We had an agreement, don't that mean anything?>_>
And by that do you mean to say you'd never do the same thing to a poor, unsuspecting AI?

The AI may not be very good at tactics, and it may not make the best choices for infrastructure improvements or even the best choices in war, but leaving your borders underdefended for too long is asking to be invaded.
And really, what better way to fool your enemy than pretending to ally with them? Germany aligned with the Soviet Union against Poland, only to turn around and backstab them. My friends have "aligned" with me in strategy board games only to prove to be filthy liars (granted, sometimes they actually are allies - have to play the honest one sometimes if anyone's to trust you in the long term). They pulled a fast one on you, and even if it wasn't actually intentional in the algorithm (only Firaxis knows that), the AI is sly enough to not always be true to agreements.
MPPs are usually bad choices if you want peace. If you want to drag several AIs into war, however, they can be used to your advantage, even without breaking your reputation. Just sign one with the AI's you want to drag into war (preferably near your border), declare war on the other AI's (preferably not on your border, but on the other AI's border) you want to be in on the war, and park a poorly-defending unit just outside the far-away AI's borders. They'll attack it, invoking the MPP, and the two (or more

) AI's will be in a big gun-slinging war, while if you played it right, you'll have to worry about the odd convoy at most. After 20 turns, make peace. It might cost you a little, but the AIs will keep fighting, destroying their democracy and thus science, and draining all their resources. Certainly not risk-free, but used correctly, MPPs can be very beneficial. Just don't sign them for the "defensive" value unless you really need the defence (and your ally would be forced to defend you from your likely enemy - i.e. they separate you from your enemy) and/or can afford a war if it does come up.
Right of passage, I use somewhat liberally. In general it helps AI relations, which at least supposedly garners you better trades. I also use it to give one AI an edge against another in wars where they must cross my territory to reach each other (while not leaving the cities along their path unguarded of course). On the other hand, there is an inherent risk (especially after railroads). And I find them generally disadvantageous when the AI is just using them to move settlers across my land. But there's a time to use them, too.