Here's why, IMHO, it is important. As Christians, we all agree that the Roman Catholic church is the "mother" church of all denominations. Now, many of those denominations don't look anything like the other. But, as St. Paul pointed out,just as a hand doesn't resemble a foot, all of Christianity can belong to the same body. Trouble is, each time we (Protestants) get close to being one body--like when Pope John-Paul I (one of the greatest, IMHO) instituted Vactican II--some thing pops up (like John-Paul II's recent pronouncements about the Rosary) to bite us on the cross. It separates us as from what Jesus wanted his holy catholic (meaning universal) church to be. Currently, the major point that separates Protestants from RC's is Marian philosophy. Now, granted, it is not as bizarre as Mormons believing in a fairy tale about an angel named Maroni selecting, of all people, a deranged individual named Joseph Smith to show golden plates to as another testament of Christ; or Jehova Witnesses's belief that only a select 144,000 may experience God's heavenly kingdom, but Marian philosophy is, in Protestant thought, strange--because there is absolutely no mention of St. Mary being emmaculately conceived, a perpetual virgin, or bodily assumed into heaven anywhere in Christian Scripture. St. Mary was simply the fully human mother of Christ and the link which allowed Christ to be human too.
Actually, you are incorrect.
As noted above, even the 16th century "reformers" themselves confessed "Mariam semper virginem":
"There have been certain folk who have wished to suggest from this passage [Matthew 1:25] that the Virgin Mary had other children than the Son of God, and that Joseph had dwelt with her later; but what folly this is! For the gospel writer did not wish to record what happened afterwards; he simply wished to make clear Joseph's obedience and to show that Joseph had been well and truly assured that it was God who had sent His angel to Mary." John Calvin, Sermon on Matthew 1:22-25, published 1562
"It is an article of faith that Mary is the Mother of the Lord and still a virgin.... Christ, we believe, came forth from a womb left perfectly intact." Martin Luther, Works of Luther, Vol. 11, pp. 319-320; Vol. 6, page 510.
"I firmly believe that Mary, according to the words of the gospel, as a pure Virgin brought forth for us the Son of God and in childbirth and after childbirth forever remained a pure, intact Virgin." Ulrich Zwingli, Zwingli Opera, Vol. 1, page 424.
"Mother"? The RC Church of today has little, or no, resemblance to the early Crhistian Church. The Church which gained it's property, wealth, and power from Constantine is not the "mother" of the Christian Church I recognize.