At Couchbase we did a survey of developers (this was ages ago) and the biggest motivator for NoSQL was schema flexibility. Not having to coordinate migrations is seen as a productivity boost. [1]
The other thing document databases can offer that relational databases struggle with is taking subsets (which we use for offline sync.) [2]
The other thing document databases can offer that relational databases struggle with is taking subsets (which we use for offline sync.) [2]
[1] http://blog.couchbase.com/nosql-adoption-survey-surprises
[2] http://developer.couchbase.com/mobile/