Oh, it's quite valid. It's valid to the story. Without them, you don't understand the characters. Without understanding the characters, you can't understand the underlying concerns...what made them who they are.
The great books are never just "great stories", per se. There's a reason why the Da Vinci Code, for example, will (or should) never be considered a "great book", even though it's a FANTASTIC story...it's because of the character dynamic. And to understand the characters, you need to understand how and WHY they think, feel and believe they do.