I noticed the documentation was missing here and added the following:
This (property) defines whether only the required attributes and associations for the object(s) are retrieved. This can sometimes improve your performance but it can also reduce performance because the objects can not be cached entirely. If you have custom widgets in your form and they need access to other fields or associations, or if your next form contains other attributes or associations of the same object(s) you should not enable this. This is why the default value is false.
Don't know for sure, but in previous versions a full object including all attributes was retrieved for a dataview. Probably you can optimize this for a form. You can check that with firebug and inspect the retrieve action for an added scheme is.