In his book Emotional Design, Donald Norman postulates that appealing objects attract users and raise their perceived value. In a nutshell:
Human emotions play a leading role in how we interpret the world. Aesthetically pleasing objects, through their delivery of sensory engagement, cause users to feel connected them. As a result, the user of a beautiful object is more likely to view the object as being effective, than the user of a less beautiful object.
Another good reason to invest in good design.