Again on the protection pact subject: the AI doesn't seem to weight correctly the implications of declaring a war against someone who has signed one:
-It doesn't seem to think very thouroughly whether it can win a war in two different fronts, against civs whose military power may be inferior individually, but not together (this might be just my imagination, though; the AI doesn't always weight correctly the military might of one civ alone either).
-It doesn't seem to care if it is on good terms with the other end of the protection pact.
IMHO, the protection pact is more than a promise to protect the other part: it should serve first and foremost as a deterrence from declaring war on either part altogether, for either of the previous reasons, even moreso than a DOF (since you know for sure that the other part is going to ripost).
Note however the italics: this is anecdotal data, so it might just have been bad luck in a couple of games.