That's a no then isn't it? If you can only think of 1 that is no where near the scale of the ones in ck2 that's a no. And you didn't answer the question about the Hammers army.
The concept of Defensive Pacts applies very well to the situation of the Crusader States and it's neighbouring islamic city-states, who formed coalitions against greather threats (e.g. Zengi had to face a pact between the Kingdom of Jerusalem and Damascus).
What could considered to be an ingame-defensive pact on a greater scale is the relationship between the Byzantine Empire and the Fatimid Caliph, which was probably directed against their mutual enemy, the Seljuks. Of course this never had any real consequences, because the Seljuks quickly crumbled after 1092 and the crusaders turned everything upside down afterwards. But that's the problem of Ck2-defensive pacts (from 2.5.2; not the coalitions from 2.5) alltogether, isn't it? They are supposed to simulate some sort of loose relationships between countries that face a similar threat, it's an "insurance" for the unlikely case, an easy possibility to disencourage further expansion, not a real coalition based on contracts and the will to actually act together against the enemy. The Fatimids and the Byzantines probably wouldn't have acted together against a renewed Seljuk expansion, despite good relationships and diplomatic contacts (hence the emperor turned to the west for help and not to the Fatimids) - but as the Seljuk sultan it would have been at your own risk, because you wouldn't have known for sure beforehand.
Similar examples can be found throughout the history of the Middle Ages - the anti-German coalition between the Byzantines, Venice, the Pope and the Normans against the actions of Barbarossa in Italy, all of them being enemies before (and afterwards); Roger II. encouraged Serbs and Magyars against Manuel I. in the context of his Italian ambitions; none of these had any real consequences and are sometimes only suggested by some sources, but the concept of Defensive Pacts is not entireley a fiction for the time. And I can easily live with them as they are implemented in the new patch - the only thing really annoying thing is the automatic call to arms. As the player you can always "backstab" the AI by simply not doing anything, but the AI will ruin itself with sending all its troops at once, even in the smallest of wars.