Pinging you as requested. :)
I gave your question some additional thought during my morning run ad have been chewing it ever since. I think the necessity for Mary to be pure had a lot to do with the cultural norms. Cleanliness was a very important aspect of Jewish culture and religion. For example only specially purified and cleansed priests could approach the Holiest of Holies in the Temple. As Mary was the ark of the infant Jesus she too had to be immaculate.