A rule of thumb is not a law. Form doesn't always follow function even in nature and evolution: http://www.youtube.com/watch?v=cO1a1Ek-HD0

Also styles and fads like "minimalism" and "flat design" are forms that do not follow function but are really forms that enforce function. Which is why they're fads and go away after enough people jump on the bandwagon and realize the fad's flaw and limitations.