http://www.teamtalk.com/football/story/0,16368,1784_4131351,00.html Found a news article with the quotes from Toshack
He's a natural defender and at Wingback he can play back far enough to play the defense successfully while still have great opportunities to bring the ball forward himself or make runs up the flank.
I don't think he's THAT great defensively. Obviously it's something he can work on, but his best moments (maybe think v Fulham last year) came when playing in a more advanced role, able to have a bit more freedom when going forward, cutting in from time to time and going to the byline as well as crossing from deep. As a left back, he is good but, to me, he seems to be a lot better when going forward, so maybe a wing back role is good for him in terms of development of his defensive ability but, right now, I would say his best position is left midfield.