This may turn out rather rambled as im writing as I think (in hurry at end of lunch break!):
Brands are rational. Even the idea of desire is rationalised.
People are not rational.
Is this not a fundamental problem with how many ads are planned. Ok, maybe you buy your insurance based on a rational basis "which is cheapest with xxx cover"; but what about your drinks, your clothes, your car? Almost certainly not.
Only the most battle scarred parent chooses their car based on what is most rational for their needs. Most people have a half rational "I need to fit five people in and have a budget of £13k", half irrational choice "I liked this one when I tested it...why? I just did."
Its one of the reasons I am always sceptical about relying on research, because research is almost totally rational... but that moment when the customer goes to purchase the product they scored 5/5 for "I would like to buy" is likely to feature at least bout of irrational thought.
Maybe irrational is the wrong word. But I hope you get my point. When I walk into a shop and choose a drink, half the time I couldnt really tell you why I picked brand a/b/c. So you really shouldnt put too much faith into the reply I give when your brand survey asks me why I just bought it.