You can get around that pretty easily though. 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).IMO. 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 there are moments all characters can contribute.
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.