These passages aren't clear. If I say to you, "My brother Albert is a musician," would you have reason to believe with any level of certainty that Albert and I shared the same mother? Albert may be my full brother, or my half brother, or my step brother, or even my brother in Christ.
As for importance of the issue, it probably doesn't make any difference in the economy of salvation. However, being mother of a big brood of kids portrays Mary as an ordinary woman. But being a perpetual virgin sort of sets her apart, makes her more remarkable. Makes it seem more appropriate to refer to her as "blessed." And despite DHK's protests and maybe yours too, saying she had other children interjects an interpretation of the Bible that had never been made before 1600. Do you think everyone who read the Bible for the first 1600 years of its existence just got it wrong?