As far as everyone participating, I agree that's a fine ideal, but not every PC is going to be able to be helpful in every situation.
I think the word
helpful is pretty fundamental here.
I talk with groups of people all the time - groups of students; groups of colleagues; groups of friends; etc. In those groups, normally some are more articulate than others. But they are not the only ones who speak. I have things I want to know from others (eg
What is it that you're finding hard about this example? or
What movie do you want to see?). I have things I want to say to others, which prompt them to respond. They have ideas and knowledge and emotions that they want to express, so they speak.
It's striking to me that, in a thread about "realistic" consequences, a defender of those is putting forward such an unrealistic picture of human interactions.
The problem with Diplomancer players is that they insist that since they are the best at talking that they are the only ones who ever talk. They do this because inevitably social encounters are ran such that PCs not adept in social skills are a detriment when they attempt to do anything. That leads to feelings that anyone else doing anything in a social encounter is sabotaging their time to shine. This is unlike every other pillar of the game.
Combat all characters are better off doing something than nothing.
Exploration, typically every character can find a way to help. Lookout for danger, navigate, scout ahead, look for food, watch for traps, etc.
Social, basically anyone but the character with the highest social skills contributing is detrimental.
So I don't really blame diplomancer players for their sentiments, the entire game tends to get ran in such a way that their feelings are only natural. I think instead maybe we focus on how the game can handle multiple players interacting in a social encounter without being a detriment.
The starting point is for the GM to think about the situation similarly to how s/he might think about a combat. For instance, why does the mad tyrant not address the barbarian or thief or whomever directly (as Eomer does to Gimili).
The next step is to think more carefully about how to adjudicate the resulting action declarations. In particular, if we take it as given that Gimli's player (ie the player of the relatively low-CHA dwarf) is more likely to fail a check than is Aragorn's player (whose paladin has at least 17 CHA!), how do we resolve this? In LotR Eomer still lends Gimli a horse, but there is an outstanding dispute between them about whether Galadriel is the most beautiful woman in Middle Earth.
Of course there are many many other ways to think about making sense of a failure in social interaction. I just point to that one because it's fairly fresh in my mind and it is the sort of thing that I don't hear much about in accounts of D&D play.
EDIT: And here we have Exhibits A and B:
At my table, if multiple characters are talking to an NPC then at the time the check is called the DM will ask, "so who wants to lead the check?", but everyone's contributions affect the DC (which in some cases might admittedly be to the group's detriment, but that's simply because that character said something that hindered the effort).
If the entire party is at the negotiation, I let all the players contribute to the conversation and ask the face-type to make the appropriate check.
This will never produce a situation in which Eomer lets the group go, and even lends them horses, but has a meaningful outstanding dispute with GImli. It flattens out all the fiction.
There are RPGing systems that do this for combat - eg Tunnels & Trolls - but D&D has never been one of them. Why flatten out social interaction when it is so easy not to. It's not as if D&D has never come up with an alternative approach that avoids such flattening out ie the skill challenge.