There's a thread on RPGNet lamenting how physical combat takes much longer than debate. The focus seems to be more on shortening combat rather than lengthening debate, but it does tie into some more things I wanted to say about reaction rolls and non-violent interaction between PCs and NPCs.
In the midst of the thread, people have brought up the example of players who aren't persuasive in real life being forced to persuade NPCs in character, then being penalized for their lack of persuasiveness, even if their characters have high charisma or should otherwise be more persuasive than the player behind the mask. There's a lot going on in the thread discussion, which I'm going to ignore. The real issue is "do you judge actions, including persuasion, on what the characters do, or how the players think it should be done?"
Ignore social actions for a moment and think about simple character actions. If a player says "I stick my head through the door," but the door is closed, do you say "you ram your head repeatedly against the closed door until your skull is cracked. Roll 3d6 and deduct it from your hit points"? You'd better not, even if you *told* the player the door was closed. The intention of the player is to look through the door; you can penalize them for looking through the door without checking for scythe traps, but you can't reinterpret their intention.
It's similar for PCs trying to persuade or intimidate or deceive NPCs. Whether the player speaks in-character or out of character is irrelevant to the game; it's a matter of personal taste. And therefore, how the player expresses the things they want their character to say is irrelevant. It's the gist of what they say that matters.
I'm sure everyone has had an experience something like this: your character is trying to get information from a guard, and you say something that could be taken as disparaging the local lord. The GM then has the guard get angry, if the guard is loyal, or laugh it off, if the guard is somewhat cynical. Whether you stuttered, or sneezed in the middle of your speech, or whatever, is irrelevant; it's the intent to disparage the lord that matters.
So my answer to the question "should you roll for social interactions, or base it on role-play?" is: yes. You role-play to check for intent, you roll for the NPC's reaction to that intent, and you modify the reaction roll based on character ability and how the NPC feels about various keywords in what the player said.