Interesting article, but it fails to address WHY Mary would need to be sinless.
So why would she need to be sinless?
I am Catholic so I believe in the Immaculate Conception. Having said that I do not think Mary needed to be sinless for the incarnation to happen. Just as I do not think God needed to become man in order to save us. Both are not so much matters of need but of rightness. Mary could have been a sinner and God could have saved us just by willing it. So why was Mary sinless? Because the sinlessness of Mary glorifies Christ and honors him more fully and perfectly. If we look upon Mary as the New Ark of the Covenant we can understand this better. It would not be right to have the Word of God carried in a defiled container. If God willed the old covenant to be carried in a holy object why not the new?
In order to receive God into her womb by the Holy Spirit.