My opinion of social mechanics, and particularly social combat, has generally been pretty low. Social skills in 4E generally encourage the players to say "I use Diplomacy!" rather than actually talking to NPCs; it also means that whoever has the best CHA skills is treated as the 'face' while all the other characters stand back and say nothing at all. The only time I've tried to run a full-on social combat system was in our abortive attempt at Spirit of the Century: the debate lasted one round and ended when one of the players pulled out a gun and shot his debating opponent in the leg.
I'd previously come to a conclusion that goes something like this: the reason we roll dice in RPGs is to simulate complicated things that we can't actually play out. For example, we don't have the skill, courage or time to actually fight each other with swords in order to resolve a combat round. However, we do have the capacity to play out a discussion or dialogue in full, so why bother simulating it with dice?
However, while playing in Zzarchov Kowalski's Blight of the Khazars campaign, I recently witnessed social combat mechanics giving rise to one of the strangest and most memorable moments I've yet seen in a roleplaying game. The setting was a village by the Red Sea where a dungeon was about to give birth to some sort of Antichrist dragon who was the son of nine (?) evil gods. A bunch of people had been possessed by Bacchus and herded into the dungeon as sacrifices, and our dwindling party was trapped in some sort of infinite maze presided over by Belial, Prince of Lies. It wouldn't be an exaggeration to say that things were looking pretty grim.
Then, after Belial showed up to speak with us, Ian's character Daerith decided that he wanted to bullshit Belial into thinking that this dragon (of which Belial himself was co-father) was going to destroy the nine gods as soon as it hatched. So away we went with the social combat system. Somehow, after a few rounds of argument, Daerith came out on top! He successfully lied to the motherfuckin' Prince of Lies himself. I think Zzarchov said that Belial needed to roll three consecutive 1s on a d20 in order to fail, and that's exactly what he did. So now Belial has told us how to slay the dragon, and we have a slim hope that the entire world will not be destroyed in the next few sessions.
My point is that if there weren't any mechanics for social combat - if the rule was just to 'roleplay it out' - then Zzarchov would probably have just said "No. It's impossible for you to sell such a preposterous lie to Belial of all people, now let's move on." But the social mechanics actually gave Daerith a small chance to succeed - the task was almost impossible, but not quite. More generally, social mechanics have the virtue of bringing something to the table that's unexpected by everyone. Just as it's interesting to see that the orc warlord randomly fumbled and dropped his sword, it could be similarly engaging when the dice randomly dictate that such-and-such an NPC really hates your guts for no particular reason (or alternatively, so-and-so has just fallen in love with you...)