This may be too simple for some to comprehend but he's younger, a better crosser, and takes a mean set piece. Having him would arguably allow RVP to remain in the box for most set pieces which makes sense due to his height. He's a better penalty taker than anyone we currently have. It's also not entirely unreasonable for Moyes to prefer a left footed left back over Fabio, especially without a left footed winger.
And what about when we need our left back to do more then cross a ball or take a set-peice? Like play quick interchanges with our wingers/midfielders, make well timed runs in behind their marker, or just generally dribble/drive the ball at their marker/defence? You know... things that full backs actually tend to do more then simply cross a ball?
Or does "simple comprehension" only stretch as far as crossing a ball... because you're right, that is an incredibly simple outlook on things.