Which is why I said these general election polls are almost worthless - they provide no data on specific areas.
Both candidates will be focusing on certain states (like MO, OH, MI, etc) from now until the election. Undoubtedly, both will alter their messages based upon the feedback they are receiving from polls.
Few points:
A.) 6 months is plenty of time to change messaging on materials/handouts based upon what they find is working. I doubt each candidate will only have one direct mailer and nothing else.
B.) Television media has some of the largest impact available, and a lot of the content going to the press can be changed on a daily basis (outside of TV ads, of course).
C.) If they agree to weekly debates (like what has been proposed), there is no doubt that their messages will change quickly (even if its only in subtle ways).
D.) They will be using the basis of what works and what doesn't work when they change their message to determine what to put on the more static direct mailers.
So, yes, polls are extremely important, and impact every aspect of the campaign over the next 6 months.