greater for speed, simplicity, and the agility of schema-less operation
Assumung we can forget about reliability we still have a trilemma: fast and agile, simple and fast, simple and agile. Document databases push the complexity from the database management system to the application code: find out every time what the current schema is and work with it and hand it over to some kind of map/reduce environment for processing. But relational databases haven't stood still either. Postgres will let you dump volatile documents in a binary-json store or even let you plugin external data sources as if they were tables (with some overhead obviously).
But the money is always going to be in transactional data.