Isn't it good for building and querying custom indexes? Doing that with mysql or postgres is very difficult or impossible depending on hosting solution.
Or would you still prefer to build on top of memcached?
VIEWS and MATERIALIZED VIEWS probably are a better version of what you are talking about?
If you are running slow queries and caching them into into a k:v store you can just do that with a TEMPORARY TABLE - sql databases are absurdly fast k:v stores.
There are valid use cases for redis in its novel types and things but my comment was trying to express that seeing that usage, in my real life experience, has been rare.