If Rudy Giuliani's illness were that serious, then we would have every reason to question whether or not he is physically capable of carrying out the duties of the nation's chief executive.
You can't have it both ways.
As a cancer survivor, I would suspect that at the time he had to make the decision, much of the prognosis, the treatment and his reaction to it were unknown.
I was only 32 when I went through surgery and treatments. I was very fortunate and the treatment was minimal. I thought I could function normally, but soon found out I was wrong.
I won't second guess Rudy for his decision regarding his cancer. Likewise I won't second guess Elizabeth Edwards. People deal with cancer in many ways.
Unless you've had cancer, you can't really judge how you would react/deal with it.