Senator Kerry's mentioning of Mary Cheney won't matter for several reasons, not the least of which is that it is a subject the republicans don't want to talk about. Check that- It's a subject that they are mortified to even have discussed in public. (Unless it's directed at stereotypical gays in San Francisco or New York, that is.)
Compare this to Zell Millers speech at the RNC convention. It was by far the most vile and confrontational speech given. But after the initial shock and outrage, it was a non-starter for dems. The evil guy was a democrat who had given Clinton's keynote speech in 1992. If the dem's attacked him, in the end, they were only attacking themselves. So let it die.
The dems may have learned something from that episode. I do not think they were trying to be cruel or were out of bounds at all, as this digby/ Andrew Sullivan stretch illustrates better than I could.
Were Kerry and Edwards as calculating as the republicans were by un-caging Zell at the RNC? Maybe. But the difference is that they were being honest, on firm moral ground and the republicans won't have anything to gain by trying to prove otherwise. That would require an open discussion. It's a non-starter.