A simple document datastore using Redis
github.com
The project provides storage for JSON documents that uses Redis as the backend data-store. JOR also provides a MongoDB-like query language for fast retrieval.
github.com
However, you have a point that if you are to use this setup, you could be better off using mongodb right away, it's a matter of trade-offs.
A use-case of JOR is, for instance, when you need to ship a system on the premises of your customer, or you have an embedded system. In such cases, the ease of configuration and maintenance of redis really pay off (we run both systems in production). You do not have all the power of MongoDB, only a fraction of it, but you also have a fraction of operations cost.
But I do agree, that if your system is big enough, or you have the resources for the proper mongodb setup, go for the real thing (mongodb) :-) JOR is some sort of middle approach, for those cases that you say, would be good to have the queries from mongo but it's just too much hassle to install it (in-house or in-premise).