He has yet to let us down defensively, but he's contributed hugely in attack. With Kuyt ahead of him, and Mascherano in midfield, there is always going to be cover for him.
Arbeloa was a good defender, but offered too little going forward. In most of our home games, he might as well not have been on the pitch. He wasn't flawless, if you recall his failure to cut out the cross for Riise's famous own goal in the Chelsea semi a couple of years ago. A lot of his play was very casual, and he lacked pace against fast wingers.
We've improved the position without doubt.