when you have arbitrary semi-structured data of limited scope to store. For example the stuff I was working on today is pluggable payment infrastructure. The vendor response is stored json (comes down the wire as xml or json, json is easier, I refuse to anticipate the structure of the data for future providers but I want the whole data returned for debug purposes), and the extra data it requires for transaction resolution is also json. Again I have no idea what this will look like for future payment providers, and this data will not result in consequences for other bits of the database.