I like reaction rolls for a particular reason, and it's exacerbated by my combat LARP experience.
Sitting in the GM's chair, in a comfy position, with a mug of steaming tea at my hand, I'm far removed from actually being in a stress situation. I know everything that's going on, I know everything that's offstage, I have unparalleled knowledge of the situation in a fashion we couldn't possibly have in real life.
But the world doesn't work like that. The fog of war is real. People don't always see what's going on, people don't always know what's going on, and people often guess wrong even if they have all the information.
In that LARP, I was one of the chief national leaders, and my character was the most powerful wizard in the game. I had to make a lot of decisions:
* Based on information that people told me, colored by their own prejudices or flawed insights.
* Based on the fact that I hadn't gotten any sleep the night before.
* Based on the fact that I hadn't gotten any sleep because the bastard, in my face and telling me I had to do something, was partying all night long two tents down the row.
* Based that I'd reinjured my chronically bad knee two hours before and it was hurting, a lot.
* Based on whether I trusted people or not, based on what I thought I could get away with politically or not.
* Based on my own prejudices, flawed insights, or whether or not I hated Soandso’s guts.
* Based on me just not knowing what I needed to know, and being forced to make a WAG.
Sometimes I got it right. Sometimes I got it wrong. Sometimes I played the odds exactly as I should have, and the odds just fell the wrong way.
Those are all variables impossible to calculate for each and every NPC. I just figure that no matter how smart someone is (or isn't), how capable someone is (or isn't), how well informed someone is (or not) ... sometimes they have bad days. Or very good days.
Heck, the last run I GMed was heavily colored by a NPC making a terrible reaction roll and thus reacting badly to the situation, no matter how much my wife's character was trying to talk sweet reason into him. Sometimes it happens. Random rolls are a good way to emulate that.
Yeah, but what if the NPC’s interacted with the party a number of times before. So what? Surely many NPCs have good days and bad ones. Heck, I'm very much a good day-bad day person. Catch me on a good day, and I'll be all accommodating about your request or inquiry. Catch me on a pain-filled day after a night of short sleep? I'm likely to be snarky, and sometimes stubbornly so.
Come to that, my first wife was even more capricious, and in all too many of our fights, I'd pull up short, incredulous that we were fighting as hard as all of that over such a petty thing, and ask "Alright, what is this really about?" Invariably, she'd pull up short, and reveal the subject which was really preying on her mind, something usually having nothing to do with me but about which I could be supportive and help defuse things.
I've kept this in mind when having a NPC snark-out, and a few times the more empathetic PCs have asked, by way of being supportive of their friend and helping to defuse things, "Hey, Nath, you seem like something's got you well off your feed. Anything it'd do you good to get off your chest?" Nice hook for a sideplot ...