Fads, my friend, fads. Someone at Apple wanted to get on the "tab" bandwagon and now they're trying to put tabs everywhere. Even where it makes no sense at all and only complicates the user interface...
Same thing with tags: some people (Evernote, among others, is notable for refusing to allow more than two hierarchical levels) can't organize their documents in a hierarchical way. Using nested folders, etc.
They invented this "new" approach where you keep everything at the same hierarchical level, but add a (or several) flag to an item (file, folder) to make it a member of a "group".
Same thing with the "look" of OS X. There was the 3D epoch where the designers tended to make things stand out (icons, for example). Now, we're entering the "flat look" epoch...
Fads have a tendency to come and go. In a while, we'll go on to something else, dreamed up be the latest "designer" to work on something...
Bah...