I'm not sure what you mean by "Mary's role was complete".
Do I believe that God "used" her and threw her away after wards? No. Do I believe that God used her and then blessed her in the eyes of other Jewish women by giving her more children? Yes.
Do I believe God honor Joseph and took away his reproach by giving him children? Yes.
This is all supported by scripture. I think Luther and Calvin didn't wish to face the "delicate" issue of dealing with such a minor doctrine. Perhaps had they known how the Catholics would have blown this up out of proportions, they might have taken it more seriously.