What are the shorcomings of any public opinion poll?:
The primary shortcoming in any public opinion poll is that there is a tendency toward bias at every stage of the questioning process.  The person asking the questions has his bias; a person who wrote the question has her bias; a person who is answering the question has a bias as well.  The former two forms of bias should ideally not show up in the results of an ethically conducted poll, however they often do.
The next most important shortcoming is that while ideally the sample set (the specific group of people to whom the poll is given) is big enough and diverse enough to represent the general population, commonly small and/or homogeneous samples are used to save money or time or both.  Sometimes homogeneous and small samples are used by unethical poll takers to influence the outcome of their poll.
In short, polls will never be 100% helpful until they are designed, executed, and administered with practically no human intervention whatsoever.  But what's the fun in that?