Kerry had a 6% lead over Bush in March 2004 (
http://www.cnn.com/2004/ALLPOLITICS/03/09/main/index.ht... ) and got the nomination and lost miserably. People say that since Obama has a lead over McCain at the current moment, he must be the nominee. But he is in the same situation as Kerry: not much is known about him, expect that he is a good speech-maker and it is much easier to distort his image to a farther extent than it is in Hillary's case. Hillary on the other hand has been in the public view for many years and there is not much new negative information that can emerge that can make her look worse, even with all the anti-Hillary press.
When people say "are you suggesting we choose the nominee who performs worse in the polls", "there is no incumbent running right now" are intellectually dishonest. Hillary has been villified for 15 years and she still holds her own. If you want to believe that Obama can hold his when he is carefully scrutinized, this is just a "belief" not supported by years of evidence.