Hey,
This system is useful for many extensions. A few things from existing extension that could make use of it from the top of my head:
* Education Program courses * UploadWizard campaigns * Semantic Forms form definitions * Surveys
And of course it facilitates creation of new extensions that want to define instances of such structured types.
Cheers
-- Jeroen De Dauw http://www.bn2vs.com Don't panic. Don't be evil. --