SS, 2B, CF are known as "up the middle" defensive positions. Typically defense is valued higher in these spots as they cover the most "fair" ground on the field and because of that get more defensive attempts over the course of a season. Power is not a must, just a plus at these spots. Inherently if your lacking at power in those "defensive" spots you look to add power in the less demanding defensive positions.
That's pretty much the basics of it. Obviously there are exceptions like Hanley, where his glove was passable at the high priority defensive position because his bat was so good.