[matc]

> The application can move straight to supporting the new structure and entirely forget that the old one existed.

Not always. Changing the schema can break an application, in particular when the database supports multiple applications.

> It's simple data and some dangling records aren't going to hurt anyone.

It depends on the application. In healthcare people literally die due to some dangling inconsistent records.

The problem in anticipating and working around these inconsistencies is that the workaround is added ad-hoc in code, rather than through a model defining the data change. You need a model http://chronicdb.com/preparing_schema_changes