Dear christian friend .... puhlease! ... Mary was immaculately conceived.
Mary's sinlessness is not for her own sake, but for Jesus' sake. She had to be holy in order to be a worthy vessel for her utterly holy Son. So this doctrine upholds the holiness and deity of Christ.
Since Jesus Himself preserved her, Mary's sinlessness depends completely upon Him. He is the Savior of all humanity, Mary included. By preserving her from all sin, Jesus shows us that He can indeed save "to the uttermost" (Hebrews 7:25).
I am not Catholic, and one of the typical non-Catholic questions is always in reference to the immaculate conception of Mary.
I finally understood the Catholic point of view when reading this very nicely-done article:
http://www.catholic.com/library/Immaculate_Conception_and_Assum.asp
"Consider an analogy: Suppose a man falls into a deep pit, and someone reaches down to pull him out. The man has been "saved" from the pit. Now imagine a woman walking along, and she too is about to topple into the pit, but at the very moment that she is to fall in, someone holds her back and prevents her. She too has been saved from the pit, but in an even better way: She was not simply taken out of the pit, she was prevented from getting stained by the mud in the first place. This is the illustration Christians have used for a thousand years to explain how Mary was saved by Christ. By receiving Christ’s grace at her conception, she had his grace applied to her before she was able to become mired in original sin and its stain.
The Catechism of the Catholic Church states that she was "redeemed in a more exalted fashion, by reason of the merits of her Son" (CCC 492). She has more reason to call God her Savior than we do, because he saved her in an even more glorious manner!"
In the Magnificat, Mary speaks of "God, my Savior." If Mary were sinless, she would have no need for a Savior. No where in the Bible does it say that anyone was sinless except Jesus.