Thomas Shey
Legend
In ancient days, there were only four human player classes, the fighting-man, the magic-user, the cleric, and the thief. Having a unique character wasn't about choosing different player options during character creation. Of course, that's how the game has evolved and it certainly is that way for many gamers today. It's pretty common to want your character to be differentiated from the rest of the party in some way, even when this creates groups that potentially break the suspension of disbelief. ("What, no humans at all in this group? And everybody's skin tone is a color not found in nature, if not covered in fur? Okay.")
Part of Hussars frustration (I believe) is not just wanting an all-knights only campaign, but the worry that a significant chunk of his playing group will want to play against type. It would be easier to roll with if it was just one player. When everybody is "the exception" then nobody is and your theme is potentially lost. While I think he favors a too restrictive game for my tastes, I get that frustration.
Again, with the right group of friends, I can see an "all fighters" campaign where the players are encouraged to differentiate by background and personality rather than in-game character options. But that isn't the expected norm, and I'd want to make sure I'm communicating clearly with my group and that I get their honest and enthusiastic buy-in before getting too invested in the idea.
Yeah, I've referred to this as the tendency for some groups to devolve every player group into, frankly, kind of a freakshow. There's certainly a tendency in modern fiction for characters to run to being special snowflakes, but its honestly kind of jarring when it applies to every or even the majority of characters in a group, and its actually easier to discourage it at all than to do what can look a lot like playing favorites.