Slava @ RethinkDB here.

Some of our biggest users (to be announced in a few weeks for 2.0) use RethinkDB this way. You can't really do deep analytics/machine learning as RethinkDB wasn't designed for that, but if you want to store a lot of data, and then run lightweight aggregation or map-reduce queries on that data, Rethink turns out to be a really good product for it.

One issue I see with this path is that if your queries ever get a lot more complex, you'd have to migrate off of RethinkDB onto Hadoop (which is a pain). I think that if you know for certain you just want lightweight querying capabilities RethinkDB can be really wonderful, but if there is a good chance you might need something deeper, it might be worth the effort to set up Hadoop early on.