There clearly is a need for something like CounchDB. All the impedance of mapping document data to relational is quite tiresome.
I don't think that we'll ever get rid of relational databases all together, but at the very least it would be nice to have something for the cases where it makes no sense to map to a relational database and you want to effectively retain your documents. This could be great just to effectively store service interactions. Is it really just JSON or can I stick XML in there too? JSON is great and all, but there are an awful lot of XML documents out there.
I thought document storage like this was a great idea going back to 2000-2001. I was working at eXcelon (now Sonic Software) at the time. They had a product named XIS that was an XML database built on top of eXcelon Objectstore. It's issue in general at the time was scalability - I am not quite sure what happened to it after Sonic Software (Progress) bought eXcelon.