You're right that MongoDB is designed from the start for horizontal scaling, and the author could have better articulated his reluctance to add machines. Howev

You're right that MongoDB is designed from the start for horizontal scaling, and the author could have better articulated his reluctance to add machines. However, his suggestions would improve mongodb performance in all configurations, not limited to single node. The mongodb guys did a great job simplfying the internal architecture and focusing on a great API to get the product out quickly. I'm confident they can make these sorts of internal improvements to broaden the use of mongodb.