I had a kind of a hilarious situation in my latest game: I wound up with my research and economic treaties tied up with the runt of the galaxy, Alcont Republic (variant humans, focused on espionage). It wasn't their fault, actually: the game settings turned out to be rare planets, rare habitable (on a huge map, the most planets any empire had was three) and they drew a bad starting position backed up in the corner behind me.
I wound up in a war with the United Mankind (variant humans, aggressive, military focus) and needed their help, but they wouldn't budge (they'd declare war on the Dread Lords, but were too wimpy to fight humans?). In fact, they started sending constructors into my territory and building influence starbases on my doorstep. This was really darn annoying, here I am fighting for my life, and my supposed friends are trying to convince my people to go join up with them.
I decide to give them the benefit of the doubt: we're pretty close, and so I try and propose an alliance, but they're too retarded to realize what that means (durr, alliance?) Actually, it was because they hadn't researched that far up the governments branch yet, and tech trading was turned off so I couldn't teach them.
It's become clear to me that these guys are a pain in the rear, and I need to take them out. The thing is, I don't want to declare war on them: my rep will suffer, and I'm a bit on the edge with the Hania Federation, (mercantilists). So once the war with the UM is over, I decide to start poking Alcont to try and get them to attack me. I put them under a trade embargo. I build military and influence starbases right on their doorstep. I mass fleets right in their territory. I start demanding money from them. Relationships degrade from close to friendly to warm to neutral. Then. . . I can't believe it, relationships rise to warm, then to friendly, and back to close. I've got these guys on trade embargo, I've got spies on literally every inch of their territory, I've got influence and military starbases right in their own darn solar system, I've got fleets of ships massing in orbit around their planet, and these guys still think I'm their friend.
I finally got sick of it, converted a couple of my constructors to Planet Killer Missiles (Spore ships, Cargo Hull, with as many engines as I can stuff in them) and launched them from my home system. I had my fleets take out their orbital pickets just as the missiles arrived. The war lasted about 48 hours. My casualties: three Resii class destroyers that were killed fighting their Frigates. Theirs: over thirty billion colonists. Half their fleet went over to Hania. Mopping up those that went to ground as pirates took about three months.
In short: don't turn off tech trading if you're even considering the possibility of a Diplomatic Victory because the AI doesn't seem to like researching governments.

Also, provoking other nations is harder than it looks. Also, despite my alignment, when it comes time to research Xeno Ethics, I'm gonna have to go Evil because, honestly, nuking a bunch of naive cwas a really bastardly thing to do (I have a mental image of the Alcont standing around looking up at the sky as the Planet Killers plummet to the ground with their deadly cargo, going, "Yay! Gifts from our good friends!"

)