I'd recommend that anyone who has doubts about social mechanics try to play some games that feature them, e.g., Burning Wheel or Spirit of the Century. I've noticed that both skeptics I've encountered online and ones in my D&D group have a mistaken impression of how such mechanics impact the game.
The simple fact is that there is no roll/role divide, and utilizing a system to resolve conflicts in a social arena in no way prevents in-character dialogue, immersion, or narrative flavor. It simply supports resolution of social situations the same way that combat rules and skill rules support those situations. As a player, you are still calling the shots and making choices for your PC, as well as interacting with the game environment. The difference is simply that dice and your PC's ability are brought into play, when there is a conflict that needs to be resolved. Obviously, if there is no conflict, there's no need to use the mechanic, just like anything else.
The following is not how social mechanics work:
Player: "I want to convince the duke to lend us troops."
DM: "Okay, roll your Diplomacy."
Player: "I got a 34."
DM: "Good. The duke lends you troops."
The following is how they might work a la Burning Wheel:
Player: "I want to convince the duke to lend us troops."
DM: "Okay, how are you going to do that? The duke currently doesn't have a very high opinion of you and your compatriots. 'Mercenaries and grave-robbers,' he says. 'That you would have the audacity to ask for my aid!'"
Player: "'My lord,' I say. 'Do not mistake us for common adventurers. We are servants of the Radiant Order, and we are worthy of your trust.'"
DM: "So you're trying to bluff him, huh?"
Player: "Well, yeah. It's mostly true, seeing as we've got a cleric of theirs in the party. I want to play that up, so I'm using Bluff with a synergy bonus from my Diplomacy to attempt an Impress maneuver."
DM: "Okay, so you're going for a full-on social combat, then? Let's set some stakes first. The duke is staunch in his resolve. 'You try my patience, peasant,' he says. If he wins, you leave his fiefdom, period."
Player: "Figures. If I win, he lends us a squad of warriors."
DM: "Sounds good. Let's start the first exchange. You said you were leading with Impress, using the skills you mentioned. The duke is in a default Rebuff stance, defending with Sense Motive, with a bonus for his superior social rank."
(Play continues, with narration, mechanical choices, and die rolls from both sides for a number of exchanges. Eventually, both sides of the debate have worn each other down, i.e. "debate damage", to point where a resolution is agreed upon.)
DM: "Okay, you've managed to reach a compromise; the duke is willing to grant your request, in part. 'I will lend you the aid of a single warrior,' he says. 'A lieutenant in my personal guard. Surely, crusaders as accomplished and blessed as yourselves will need no more than the aid of one knight.'"
Player: "Crap! Not quite what I'd hoped, but it's something. 'My gracious lord,' I say. 'Your generosity is most humbling. I feel confident we will triumph now that we are given the aid of your champion.'"
DM: "Hehe. Okay, so, the next morning you go to the barracks to meet the lieutenant..."
And there you go. Now, one could argue that this could have been handled sans-rules, but the point is that the mechanics helped push the story in directions neither side may have envisioned at the outset. I.e., in this example, neither the DM nor the player had any idea how the debate was going to end.
Without the system, there'd really be nothing to prevent the DM from just shutting down the player's idea completely, or alternately, nothing to prevent a really gregarious, forceful player from bullying the DM into giving them whatever they wanted, even if their PC was a Cha 6 half-orc barbarian.
Not to mention, if it made perfect sense for the duke to help the party, there would have been no need for the social combat. The DM would have just said, "yes."
I dunno. I find the above a lot more enjoyable than the typical Mother-May-I of pure Player-DM negotiation. It also gives players a reason to care about non-combat abilities.