* A wider stance is good because in addition to promoting a stronger shoulder turn, it encourages your lower body to temper excessive hip movement. As a result of this setup, you are likely to put more concentration into the core muscles and upper body, producing a shallow approach to the ball that leads to more accurate contact, straighter ball flight, more spin and (simply because you are achieving square contact) more consistent power and distance with the longest clubs in the bag.
* A wider stance is bad because in addition to encouraging you to sway, it restricts power-inducing hip speed. The result is a weak shot from a glancing blow that often produces a push slice.
The second one tends to be more true of a stance that is too wide. You're right, the definition of "wide" matters.
there's a happy medium that's not too narrow and not too wide. making a full swing with a narrow stance can make you lose balance, and making a full swing with a wider stance can restrict your movement, just find what's comfortable for you and stick with it