Why do you expect the decision made in a default state to be more accurate than if it's modified by an unknown factor? If you don't expect to be primed in any particular direction, you may as well be swayed towards the right choice.

Besides Yvain's point about noise, I would point out that we live in a society that systematically seeks to distort our decisions toward non-optimal decisions. I refer to the >$150 billion advertising industry.

I certainly don't believe that a supermarket's many careful product placements and signs and illustrations are priming me to shop more rationally than I would without those stimuli.