Too Old And Sick? Or Too Conservative And Hypocritical?
Here's the thing: John McCain is hardly a newcomer to the political stage. He's been a member of Congress for 26 years -- and an extremely prominent one, too (how many people can even name Arizona's other senator*?). He's been campaigning rather energetically for the past year. So we have plenty of relatively direct information about how he'd govern. So there's no need to make tenuous inferences from his health records.
I'm not saying this because I love McCain -- I voted against him in 2006, will vote against him in November, and assuming he loses then (and I'm still living in AZ), I'll vote against him again in 2012. But I'm not voting against him because he's old and might get skin cancer. I'm voting against him because he's pro-war, anti-soldier, pro-torture, anti-immigrant, anti-environment, anti-abortion, anti-GLBT, pro-corporation, and a bunch of other things of that nature.
Making a big deal over McCain's health reinforces the idea that old people are frail and unable to contribute to society, that we should judge someone's moral worth by the pronouncements of the medical establishment, that people with health problems can't be relied on to have significant responsibilities, that any views or behaviors we don't like in someone else should be attributed to mental illness. That's not cool, even when the person you're saying these things about is loathsome and would make an awful president.
*Jon Kyl, who is even worse than McCain -- he's just quieter about it.