A gazillion already-made programs (for example data visualization tools, analytics software, etc.) speak only SQL. If the choice is between reinventing the wheel for each NoSQL DB vs. using a SQL DB (such as http://luciddb.org/ [1] and soon postgres with their recent introduction of foreign data wrappers[2]) as an intermediate layer or even a full data mart that does what you want query-wise, a lot of people want to save time and just use the SQL DB. (Why do you think Hive exists?) There's more to DBs than storing user data for cat picture sharing sites...
[1] Disclaimer: I wrote a preliminary CouchDB connector for Lucid that'll get released separately with the upcoming 0.9.4 release.
[2] I believe one of the first postgres foreign data wrappers was around Twitter. So you move some API accessing code from an application layer that realistically only your application can talk to, to the DB layer, and now everything that can talk to the DB layer can access the data. (Unless they want it raw, then they'll get it from Twitter.)
[1] Disclaimer: I wrote a preliminary CouchDB connector for Lucid that'll get released separately with the upcoming 0.9.4 release.
[2] I believe one of the first postgres foreign data wrappers was around Twitter. So you move some API accessing code from an application layer that realistically only your application can talk to, to the DB layer, and now everything that can talk to the DB layer can access the data. (Unless they want it raw, then they'll get it from Twitter.)