It's not. Though sure an amazing number of gamers will say the game MUST ONLY be played this ONE way.I would think this sort of mix is nigh-universal. The stereotypical party of four has one sneak (a Rogue-type), one quiet person without sneaking skills (a Wizard-type), one probably-armoured person who can't sneak well (a Cleric-type) and one certainly-armoured person who can't sneak worth a damn (a Fighter-type). Rare would be the party of all sneaks and rarer yet would be the party with none.
And I have a special loathe for the Bully Four players. The type that picks the class/race/character THEY want to play...and then Bullies or Forces another player to be one of "The Four" that is left.
It's a quick way to get booted from my game.