Oh indeed, most RDBMSs offer great flexibility. They just tend to require a lot of configuration work. My impression of the NoSQL wave was "we don't know and we don't care; just throw it all in there and let search sort it out."

In fact, that mentality originated with Google (and it's entirely a bad way of doing things). Managing, categorizing, classifying, etc. in non-trivial cases may be an effort of relative futility, constantly chasing a moving target.

A simple key/value or document store just lets you start storing before you may actually know what you're storing or what the patterns and structures are (or even what you might want to do with the data later). With RDBMS, your designs can change quite a lot based on these requirements or intended uses.