Don't confuse the "RDBMS uber alles!" for all storage is equivalent to "RDBMS is always a fail." Certainly over the last decade or so there was a mistaken belief that all data should be in an RDBMS, even though they were flattened or required no relational algebra, but that doesn't discount the benefit of a tool that can handle such relations when necessary.
Redis solves a certain set of problems for me, Memcached as well. ElasticSearch solves another set, and RDBMS yet another. Square pegs for square holes, round pegs for round holes.