Think about it in terms of your own life. Would you like to be tied to all the political views, bad judgments, or questionable actions of all your friends and family? How about with people with whom you may have sat on a not-for-profit board? You know, those people you saw maybe once every month or two, assuming you both attended all the meetings - people who you hardly knew other than to recognize them on the street. The result is you could be accused of exercising bad judgment for supporting a worthy cause. Nice. This is precisely the reason that a larger number of talented citizens don't run for public office.
The good news is that the more McCain "Ayers it out," the more the lead widens for Obama. That's not only good news for Obama, but with any luck, it's good for the future of American politics.