"The Most Holy Virgin Mary was, in the first moment of her conception, by a unique gift of grace and privilege of Almighty God, in view of the merits of Jesus Christ, the Redeemer of mankind, preserved free from all stain of original sin." (Pope Pius IX, Ineffabilis Deus, 1854)
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).
On the other hand, were Jesus born of a sinner as you claim, that would mean either
a) the Savior could not save His own Mother from contracting sin or
b) He refused to honor His Mother in every way he could.
So Jesus would be either a weak Savior or a trangressor of His own Commandment to "Honor your father and your mother" (Exodus 20:12). The Immaculate Conception is clearly a belief which exalts Jesus!
You're gonna take the word of some pope over the Word of God? There's no point in continuing this discussion...
Ro 3:23 For all have sinned, and come short of the glory of God; (KJV. Emphasis mine)
God trumps pope. Mary was a sinner. If she wasn't then why did Jesus need to come here at all? Mary could have died for our sins if she was perfect. The fact that Jesus came to earth to die tells us that no one else was perfect enough to be that sacrifice