She is a woman, she's reasonably young and attractive, which will sway more people than you'd think. She appeals to the group that McCain SORELY needs and is not overly popular with, the religious right, because she is 100% pro life (and pro guns, which doesn't hurt). Like TSD said, that group is thrilled she has a 17 year old PG daughter who is getting married and raising the baby - she kept the baby, didn't even consider abortion (same with Palin's child who has Down's Syndrome.)
I don't think McCain is that foolish to belive she would get Hillary voters (though the press wants you to believe that). This woman is just the Anti-Hillary.