Why have a slow database with a caching system to support it, when you can have a fast database that can respond in less than one millisecond under any transaction load?
Yes, because there are no imaginable criteria for a database rather than latency.
"Here's my hammer. Notice how every problem is a nail?"
The Redis team should be interested in my new NoSQL database, 14base. While it doesn't guarantee the correct answer, unless that answer is "14", it is exceedingly fast. It uses a proprietary search function which returns "14" in response to all queries.