An application that appears cluttered or illogical is harder to understand and use.

Aesthetic integrity is not a measure of how beautifully your application is decorated. It’s a measure of how well the appearance of your application integrates with its function. For example, a productivity application should keep decorative elements subtle and in the background, while giving prominence to the task by providing standard controls and behaviors.

An immersive application is at the other end of the spectrum, and users expect a beautiful appearance that promises fun and discovery. Although an immersive application tends to be focused on providing diversion, however, its appearance still needs to integrate with the task. Be sure you design the user interface elements of such an application carefully, so that they provide an internally consistent experience.

From Apple’s iPhone Human Interface Guidelines

http://developer.apple.com/iphone/library/documentation/UserExperience/Conceptual/MobileHIG/PrinciplesAndCharacteristics/chapter_4_section_7.html#//apple_ref/doc/uid/TP40006556-CH7-SW7.