A cross is a "religious decoration", says Rowan Williams, the one he is wearing might well be! Though I think here he is wearing an Orthodox Panagia from his jewellery box - not quite the normal thing for a western bishop or even "bishop". Of course he comes from a form of Protestantism that has had difficulty with icons and Crosses and although he has written about iconography I suspect he doesn't have a visceral understanding of the meaning or attachment to such things.
The Cross I am wearing is not a decoration, I do not have a collection of them. Mine is a sacramental, and therefore an aid to holiness, it is part of my Lenten discipline, irritatingly heavy enough to remind me to pray. You could say it is in part the fruit of a vow, all be it a personal one.
He is right no Christian is under an obligation to wear a cross but religion today is about personal choice and I choose to wear a Cross. If I were a Coptic Orthodox I might choose to be tattooed with Holy Cross or in a former age to branded with the Cross.
Nothing obliges a married person to wear a wedding ring but it is a sign of devotion and love, it is the same as my Cross.