I know for shimano and campy, there are generally 5 grades in quality: sora -> tiagra -> 105 -> ultegra -> durace. But I have heard that for 105 and up, the components are actually less durable and are more geared toward light weight and speed. Is this true? How much weight are we talking about?

Also, there are other components such as wheel hubs and bottom brackets that aren't named in this grade system. Is it just because they're generally the same, and there's not much to distinguish?