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As I recall, the argument says that the tool is so heavily used by webmasters checking for those killer keywords that the results are not a true reflection of searches by Joe Public.
I have no personal knowledge of this or any particular viewpoint.... just passing on info from the dark and often dusty corners of my memory.
Because this querying may be automated, you sometimes see a very strange seven word phrase that has been searched on an unlikely 60 - 100 times.
I think GoTo has a challenge to meet here. If they can resolve it, they will grow. Since they seemed to have solved the much bigger challenge of "competitive clicking", I look forward to how they deal with artificially inflated search counts.