Here's an example of why advanced defensive metrics are much better in judging defensive value than fielding percentage (via Deadspin.com):
That is Tigers shortstop -- and defensive whiz -- Jose Iglesias flashing his outstanding range and making a ridiculous catch in left field. But if you only look at the play by play log, you'll see this:
David Ortiz :Strike looking, Ball, Ball, Strike swinging, Ortiz popped out to shortstop.
Had Iglesias missed the ball, it would have been called a single, not an error.