We have an organization full of analysts who know SQL very well and write complex queries. Moving them all to a mapreduce query language proprietary to mongo, riak, or cassandra (and then being locked into it) would have been much more painful. Postgres has worked out very well, and I had confidence in it from prior experience which influenced the decision. With proper partioning in the schema and a json type we have the best of both worlds (SQL+NoSQL) as far as this system needs.
To get the same flexibility you'd want from a nosql data store. Without giving up the simpler, more open access methods. The schema is partly predictable (and indexed or fielded on those parts) and partly unpredictable.
