It depends on what you want. Frosting alone can be quite beautiful, but it's difficult to make in large quantities and keep the color consistent - the color gets quite streaky. [Note: the person behind the keyboard actually hates fondant and thinks this statement is bullshit. For the record.] Fondant gives a lovely smooth look to a cake and is better when you want intricate details. You couldn't make the bird I showed you earlier out of frosting as easily, for example.