David Luiz isn't all that tall, 6'2", but he sure is comfortable on the ball. And the boy can jump, pass, dribble and shoot. A midfielder in defense. If his mental abilities improve, he will have a bright future in the EPL.
Thiago Silva is only 6', yet he is in my opinion one of the most promising defenders in the world. Can pass the ball and fast as hell. Some consider him player of the year in Milan, last season.
We already have a tall boy in defense, VK. To make the defense more versatile, a technically gifted, pacey defender would complement VK much better than a typical EPL snail paced Colossus with two left feet.
Personally I think Mertesacker could spell disaster for Arsenal, given his lack of pace. If Arsenal play the offside trap, Mertesacker will be a liability.