I disagree. I don't think arbitrariness is a necessary outcome and clearly a DM should be fair and consistent when making rulings. Those rulings might be based on rules or they might not be.
That's a poor argument. "Doesn't necessarily" isn't a sufficient defense against the charge that a fluid interpretation of the rules lends itself to arbitrary judgements.
I would say they serve the DM and the DM uses them in his or her role to help the group achieve the goals of play.
I'm saying that absolute DM fiat, without baseline assumptions, does not serve the goals of play. The goals of play are best served when the players have reasonable expectations of what outcomes are possible. Your argument that the rules are fluid and entirely subject to DM fiat at all times does not establish a useful baseline.
I'm not suggesting it - I'm saying it outright because I believe it's true based on my reading of the rules. The only limit is that the DM can't describe what the players want to do, including how they act, what they think, and what they say. Players control that part of the conversation. The DM controls the rest.
Special pleading. You've established that all rules are subject to DM fiat, but the one rule about players determining what their players do is exempt from that fiat. You haven't established why that one rule is sacrosanct.
It's me saying I think, based on your comments, that you view the game as if it were a previous edition which is a root cause of our disagreement.
You have no idea how I view the game, and it's certainly not like it's a previous edition. I further find it ludicrous that you think that this edition somehow empowered the DM more than previous editions did. Granted, this one makes it more obvious, but it's not special in the rule zero case.
I do view the rules as presented as the baseline assumption of the game that my players will bring. I don't think they should be changed or viewed as fluid because that prevents the players from making rational choices based on expectations. That's not to say they can't change, or that the DM isn't responsible for making rulings when the rules fail to cover a situation, but that the DM should not be changing rules willy-nilly or on a whim. Your blanket argument that the rules serve the DM result in arbitrary enforcement of rules as a supported playstyle. Why, then, have a set of rules if your first inclination is to ignore them?
No, it is not an accusation of "badwrong." See my comment above.
Yes, it is. You told me I was doing it wrong, that I my choices (which you don't know) don't fit an RPG. That's an argument from a narrow point of view, and it's incredibly smug and elitist.
The rules say that a 10th-level barbarian can frighten someone with his or her menacing presence. Then there are some mechanics provided that the DM can choose to use to resolve uncertainty in that effort - uncertainty that the DM establishes, not the rules. The DM narrates the result of the adventurers' actions per the basic conversation of the game. If I as DM base that narration on how the targeted player thinks the action will turn out, I am still following the rules.
No, the rules establish that if you use the ability, the target makes a save. The save is a roll of the dice, not some other mechanic. There's nothing in the rules that say, "determine if the DM thinks there's conflict when you use this ability before rolling your save. Depending on that determination, anything the DM wishes may happen." You're inserting this concept of uncertainty when and where you want to to justify ignoring the rules as written. Again, you can do this, but please stop pretending that there's a 'determine if there's uncertainty' step in the PHB or DMG. That's your rule, and you apply it how you want to, and that's perfectly fine. But it's not part of the baseline rules at all.
Not wrong, but it fits with the correlation I'm sensing with regard to those who disagree with my position. Many of those who disagree with me let their players control when ability checks are made as if they are powers that can be activated. I don't think that is what the rules intend, even though it's certainly a common way for people to play. I think that method arose in 3e and 4e (both games I play and like), but is not compatible with D&D 5e as I see it.
Special pleading, again. You can't just say that 5e is exceptional in this regard as if it actually supports your position to do so. It may be special, it may support your position, but you can't get there through assertion. There's nothing in 5e that, in any way, explicitly establishes that your choices are correct. The way that Intimidating Presence works seems to contradict it, at least as far as you choosing to ignore the explicit functioning of that ability. I get that you have a mindset and a method that works for you and yours, but it is not inherently better than another's just because you think it is. Nor is it more correct. You choose to add player immunity to social abilities as you wish, but that's not inherently better than allowing those abilities to work on PCs. Nor is either position clearly supported in the rules -- they both can claim equal precedence (your ruling on Intimidating Presence, though...). You've been smug and making assumptions about how people do things at their table with the implication that if they just opened their minds and agreed with you, their games would become better. I know what you're doing, I fully understand it, I do many similar things at my table (I give my players great agency with their characters to determine things about the world and to engage as they choose), but I reject the idea that the game constructs of characters are somehow uniquely sacrosanct just because there are players behind them.
To me, if the player asks me 'I'd like to make an insight check to see if my character thinks this guy is telling the truth,' I don't see anything wrong with that because that's the player wanting to tap into the character's abilities and awareness. I'll allow the roll, and, based on the result, tell the player what their character thinks -- 'yep, this guy is shady as heck' or 'he seems pretty honest to you'. The player can then decide what they want the character to do with that. It seems to me that you are less willing to allow the players to tap into their character's abilities and awareness, instead preferring to leave the interaction entirely at the roleplaying level, escalating such die rolls to crucial points of conflict in the scene. I don't see much point in that, I'd prefer the decisions the players make on the information to be the crux of the scene, not the die roll to resolve uncertainty.