In general, skill rolls in D&D mostly go towards:
Lore checks (Nature, History, Religion, Arcana) and is mostly to determine what your character would know about a thing. (As a long time D&D player, this partly how I determine how much of my knowledge of the game to consider in-character knowledge)
Checks regarding physical challenges like climbing/jumping/etc.
Social interaction, where you generally go to the dice when an argument is such that a particular NPC might go either way. If a player presents a reasonable in-character argument then you might not need the dice, but there's a chance the NPC might not react as the PCs want then you bring the dice in. You might also use the dice if a player isn't sure what to say so the dice act as a crutch for when someone is feeling unconfident about RPing or just tired or whatever reason.
Point is, in general, RP/player choice is priority over dice in social interaction. There are going to be places where a failed die roll results in a terrible thing, but usually you only get to such a point because of successive player decisions.
Which is why I prefer CRPG dialogue trees based die-rolls to function for giving you hints or opening up extra rewards, but not being the primary success/fail of the core plot-element of the dialogue. Which is why I HATE the Kahga dialogue. It should be a puzzle of navigating responses rather than "roll this die and hope the computer is feeling friendly." You've made no decisions to bring you to this point. Your choices or thoughts don't matter. You just have a die roll shoved into your face.
Not to put too fine a point on it, but you counter your closing argument in this post. Your given example is trying to convince the NPC that she should just let the thief go. The thief, meanwhile, didn't just nab a few apples, or a few coins, but an idol to the temple's deity. Why shouldn't this be a really hard check? One side thinks that it's harsh, and remember, at this point, the goal is to just lock the thief up, which the other side feels is justifiable. The thief panics, and that results in what happens if you fail the roll. As it stands, I'm 50/50 on it, I've passed it once, and failed it once. The point of the roll, however, is to get the NPC to go against what they think is the right thing to do. Why should this be easy? It's not even that I'm a fan of the outcome of failure, it's just that I do understand what I'm trying to do when I decide to roll that die. A more lenient DM may let that conversation play out for an hour, but pacing is a thing in video games, and if we want every conversation to play out 100% in our favor every time, why not just do it on the first line of dialog, and be done with it?
This should be a hard check, because we're trying to assert our will on the NPC. We did, in fact, make a choice, we chose to interject and try to assert our will on the outcome. Things like this are what add replay-ability to games. "my next character will get this" sort of scenarios, and I don't take issue with that. I'm looking to spend thousands of hours in this game, after release. Things like this will make that easier to do, so I hope there are a lot of things like this going on as the game progresses.
