I think design docs are good for three things: giving people an overview-understanding of the game, providing detailed information for someone working on a specific aspect of the game, and making publishers feel loved and comfortable through their sheer volume. Over the years, I've come to quite resent the last of the three uses, and fight tooth and nail to not provide it. Documents are time consuming and inflexible, period, and neither the publisher nor team makes effective use of the giant document, especially when game design needs to be agile and flexible.
My time is best spent on points one and two. I like main design documents that only provide an overview, including detail about intentions and goals and how they inform design systems, and a summary of play mechanics (I prefer in the style of a game manual). Additional design documents are only produced as needed, since detail is costly but sometimes necessary, and should only be provided to head off work that needs it.