I believe that less is more because the message that is being communicated is done so in a clearer, cleaner fashion. If you ask someone that isn't a designer to name a design they know of that sticks in their minds they automatically think of designs such as the McDonald's or Target logo. That's because a design that is less busy is more easily read by all types of people. It is less confusing when a designer uses the least amount of text, color, and graphics as possible to successfully communicate a message in a design. The more unneccesary material used in a design only creates more time for the target audience to process what the design is really about and who it is targeted at and it allows more time for the viewer to become uninterested or distracted with something else.
At the same time though, it is crucial for a designer to use the appropriate amount of text, color, graphics, and creativity to set themselves apart from what is already offered to the public for something new and fresh to look at.
A good design has a purpose or meaning that is communicated as clearly as possible with as little distraction than what is absolutely vital for success. People are drawn to what is pleasing to their eyes and generally a clean, non-noisy design is much more appealing to the eye than a busy, cluttered design.