I'm not going to get into whether any rational progressive could think it makes sense to prefer John McCain in the White House to having it occupied by Hillary Clinton's virtually ideologically indistinguishable colleague. But one thing that should be said is that focusing entirely on Roe v. Wade as a reason to oppose third-party narcissism is very mistaken. Yes, it's true that replacing John Paul Stevens and/or Ruth Bader Ginsburg with a Republican appointee will be bad for abortion rights, although this is likely to occur by further draining content from Casey rather than overturning Roe outright.
But even when it comes to women's rights, this is just the tip of the iceberg. The enforcement of civil rights protections for women is likely to be much less in a Republican administration, for example. The global gag order will remain firmly in place. And in general, four more years of a tax-cut-supporting, massive-defense-spending GOP president will not only make any kind of serious progressive reform (much of which disproportionately benefits women even if not specifically targeted to do so) virtually impossible for four more years but will also make it more difficult in the future. A McCain presidency would be very, very bad for women even if not a single Supreme Court vacancy opens up during his tenure.