I have run in many shoes varying in mass. I can tell when my shoes are heavy (like my Nike Vomeros from ages ago) because it is harder to run in them in general. It is so much easier to run in Vibram Fivefingers or racing flats since they are so light. I know that racing shoes are very light because they are meant to run fast in, but why only on race day? Wouldn't running in light shoes all the time help us gain more benefit from being able to perform better during training?