I think Vogel's advice loses something in the phrasing. Having multiple difficulties is important, but making the "default" mode the easy one encourages the developers to design with the easy version of the game in mind. this is important in strategy/RPG/puzzle games, where the difficulty corresponds directly to how much thinking is involved. If you start with the easy mode and then tack on the hard mode after the actual game mechanics and content are set, the hard mode will usually just end up as a set of statistical skews and extra "hard quests" that are quantitatively more demanding, but don't leave you with any qualitative feeling of added difficulty, only of frustration. For examples, think of the speed increases in Tetris, or the Weapons in FF7.
By comparison, if you design in difficulty from the start, the difficulty will be interwoven with the game design. You can always make the game easier later, but you'll still have the areas that require Advanced Thinking to surmount—you'll just have a shortcut available as well, if you wish. For one example, the Mario series is designed such that every stage can be beaten as "small" Mario. Every power-up is, in ways, a shortcut—if you want to experience the game at its cleverest, play without them.
By comparison, if you design in difficulty from the start, the difficulty will be interwoven with the game design. You can always make the game easier later, but you'll still have the areas that require Advanced Thinking to surmount—you'll just have a shortcut available as well, if you wish. For one example, the Mario series is designed such that every stage can be beaten as "small" Mario. Every power-up is, in ways, a shortcut—if you want to experience the game at its cleverest, play without them.