The challenge with em units is that if you modify a parent element's size it changes the children as well. In your example, if you put 'font-size: 2.0em' on the .something class then the .inner would end up changing to 3.0em. That's rarely what you actually want to happen (especially in a web app). If you just want to control the overall font scaling of everything on the page using rem is preferable, because then they'd ignore the parent and scale based on the root element size.
The key is to use rem for font-size and em or unitless for all other properties: padding, border-width, margin, line-height. That way the size of the element is independent of the context.
