Good aesthetics are more than just fluff when it comes to design. They are a core part of a product's functionality. Such is the argument Donald A. Norman makes in his insightful 2005 book Emotional Design: Why We Love (or Hate) Everyday Things. For Norman, attractive things work better by boosting the mood of the user and therefore allowing him or her to think more clearly and operate it more efficiently.
Undergirding Norman's thesis that aesthetics directly influence operability is his distinction between the three basic levels of human cognition: the visceral (jumping at a sudden sound in a quiet room), the behavioral (relaxing in the solitude of a quiet room) and the reflective (thinking to oneself about why a quiet room is more enjoyable). As Norman asserts, these three levels of thought processing "interact with one another, each modulating the others" (7). You cannot escape the effect that one level of thought processing has on the other. As such, a visceral reaction to an external stimuli influences the subsequent behavioral reactions we have, which in turn influence our reflective conclusions about the stimuli itself. If we have a negative visceral reaction to a poorly design website, our mood is negatively affected in such a way that hinders our ability to navigate and use the site, even if there's nothing wrong with the navigation or user interface from a technical standpoint. All our brain can focus on is the poor design. This reaction is similar to the way humans form first impressions of others; if an individual makes a poor first impression (a visceral reaction), we are less likely to act on his or her future actions or speech (a behavioral reaction), which in turn affects the entire way we think about that person (a reflective reaction).