Mary, Mary Quite Contrary

[A small addition has been made to this post to clarify a point. – Funky]

I have a question for my fellow Catholics on this the Feast of the Immaculate Conception

Why did the Church find it necessary to define the Marian dogmas as such? Shouldn’t dogma be limited to those articles of faith which are absolutely required for salvation? As far as I know, no authentic orthodox Christian church requires belief in the Marian dogmas for salvation. Though Tradition can supplement and even interpret Scripture, it cannot contradict Scripture. Scripture states quite clearly that faith in Jesus Christ is what’s needed for salvation. One could also argue that participation in the sacraments is needed, too, but that’s another post. Put succinctly, if it’s not in the accepted creeds, it’s not, strictly speaking, necessary, though it may be appropriate or even laudable.

My faith is not affected, for good or ill, by whether or not Mary was conceived without sin. Nor is it affected by her bodily assumption into Heaven. I accept these dogmas as a faithful and obedient Catholic, but I do not understand why they are important.

Could someone please explain why they were defined? In particular, why couldn’t they remain at the level of doctrine rather than be promoted to dogma? I’ve heard that the Orthodox agree that Mary was conceived without sin, but were uncomfortable about declaring as dogma. Since the Orthodox Church is the only other Church we recognize as maintaining the Deposit of Faith, their discomfort gives me pause.

On a related note, where does the tradition of Mary’s perpetual virginity come from? I know the words for "brother" and "sister" in Scripture can mean "cousin" or other relatives, but what evidential support do we have for this interpretation?