I think people are getting a bit overly emotional about his issue right now. I can totally see where the secret fire is coming from. If a player takes no effort to role play - and like he said, we're not talking rhetoric here - when other players are willing to make the effort then I think the game suffers for it.
And here, I'm not talking about always talking in character or with flowery speeches that exactly fit their Charismas. I'm looking for an effort to put together the major points of an attempt at diplomacy, the tactics used, and all fitting with the PC's perspective and what they could and would understand of the situation. If the player can do it in character, all the better.
But if someone wants to just say "I use diplomacy on him" and roll... they can do that at another table.
Bang on. And people like that exist, regardless of the protestations certain people are (poorly) attempting to make.
I don't understand why it is hard for you to accept that this category of people exists.
For our group, we expect at least some effort. If a player can make their own decisions on how they move on the battlefield or what spell to use (even if they're not tactical geniuses), then they can put at least a bit of effort in explaining how they're going to use Diplomacy. (Any suggestion that this requires "method acting" is a silly strawman. Stop it if you want to retain a shred of credibility.)
By allowing players to make their own decisions of how their characters will act in battle, they've already made the concession that they are accepting players' abilities will impact their characters' abilities. Any suggestion otherwise is demonstrably false. So let's not go there, either.
I understand different groups may want to play differently - play styles vary widely, and some people may not be a good fit for certain groups. A shy player (to the degree of not being able to come up with a reasonable effort when using Diplomacy) insisting on playing a "face" character in our particular group probably wouldn't last long, since they just wouldn't be a good fit in our group's long-since-set dynamics and expectations (as to what we consider fun for us), just as a player who is consistently lousy at complicated rules probably would be encouraged to not play a wizard. (Thankfully, nobody is shy or have poor rules knowledge in our group.) That's just the way things go sometimes.
(Our group and I also don't make the mistake of conflating and comparing "physical" skills to "mental" skills in a "mental-based" game - so any of those comparisons are immediately dismissed as nonsensical. We're not at a LARP - we're sitting around a table making wholly 'mental' decisions. We absolutely make that distinction.)