That was my original point 200+ posts ago, but you said it much more eloquently than me. A lot of times, bluffing your way past the guard is far easier than using a charm person spell or similar, where at least the guard would have a saving throw or a will defense. Plus, a lot of people think using Diplomacy or Bluff is essentially like a dominate person or charm person effect.
In the case of the gatehouse guard, the PCs chose the wrong time and target. Posing as diplomats had too many catches, higher DC and probably get you passed up the chain.
Posing as turnip delivery = lower DC and probably get you into the kitchen and ignored during the hustle and bustle.
In real life, it REALLY is that easy to get past the right guard with the right simple story.
I assume most folks don't want the rules to allow a ridiculous lie told to the wrong NPC to be allowed to work because the PC went all munchkin on his social skills.
There's a castle with 2 locked doors. One leads into the quiet kitchen, where nobody's at because supper is over. The other leads to the guard house before rest of the castle, which is chock full of guards. The kitchen lock is of poor quality and easy to pick.
If the PC goes for the kitchen door, it's easy to get in (roll a succes).
If the PC goes for the guard house door, it's a harder lock and IF he picks it, he just walked into a room full of guards.
Assuming the PC is a master lockpicker, one door leads to success, one door leads to more trouble (failure). Who's at fault? The GM or the PC?
This example is intened to be a parallel to the social attempt to Bluff the guard. If the PC goes to the harder door, I think the player has some culpability. If the GM negated any means of gaining intel (refusing any attempts and making both doors of equal nature), then it's the GM's fault.
As to should the Bluff not engender belief, but also action, it probably depends on the nature of the presented truth. Certainly, that's what the liar intends to happen. When I bluff in poker, I am trying to get you to react as though you KNOW I have the cards I am PRETENDING to have. When I lie to my parents, I am trying to NOT get punished for doing what i did.
A case could be argued, that if the guard's reaction to somebody showing up at the gate is to call his supervisor regardless, then he is in effect, unbluffable. If his reaction is identical to whether he believes or not, then the roll was a waste of time. If the difference is that when the boss shows up and he BELIEVES the lie, then how he presents them to his boss might affect the bluff to the supervisor. If I show up at the gate and my underling says "these guys CLAIM to be the diplomats" versus, "these diplomats showed up, their papers appear to be in order". That subtle difference acts as an influence on the supervisor who makes the final call.
and this is where more DM fiat comes in, the supervisor might believe the bluff, but be stuck with the 'no entry after 10pm" rule. He might say, "hey, I'm sorry but we're not permitting entry after 10PM. I know, it's late, you guys are tired. Here's a note to the innkeeper at the Weary Arms. He'll put you guys up for the night, and if you come back tomorrow and ask for me, I'll make sure your not hassled again."
Here's what happened:
I just denied immediate access to the castle, as that wouldn't make sense
I left an open for them to get in tomorrow with minimal fuss
I did not escalate this further up the chain to cause more skill checks (which one will inevitably fail).
The ball is still in the PCs court. the skill checks suceeded, so no alarms have been raised. The party has a chance to back out or move forward. If the party has an urgent need to get in TONIGHT, once they leave the gate, they are free to pursue a new strategy.
On the topic of DM Fiat, which gets thrown around like a swear word, I think its important for all GMs to realize that they are making this stuff up. every bit of it. Whether you wrote it down before the game, or made it up on the spot, it is made up. And while the consequences for any PC action might seem logical, and in a way they are, for each action, there are a multitude of logical and varying responses.
Personally, I believe responses that make things harder or more complicated should be used when the PCs make a mistake or choose poorly. it usually beats outright killing them, as well. I also use them as a plot device, when what I've designed was deliberately 'simple' and the complication is what makes it level appropriate. I'm a bit wary of making situations complicated for the sake of being complicated, as I find making it too hard is too easy.