If the document has neither type and creator codes nor a filename extension, the Finder puts up a dialog box stating that there is no application available to open the document, and offers the user a chance to choose an application to use temporarily. Choosing an application to open the unknown file does not add a filename extension or type and creator codes to the unknown file. Apple recommends that abstract types be copyrighted to ensure uniqueness. Editor - The application can read, manipulate, and save the document type. Viewer - The application can read the document type and present the data it contains.
Applications enjoy total freedom to exist on any type of file system.
And remember that application attributes do not constantly have to be re-read from within the application itself. They are cached in a centralized, per-use database by the Finder. The situation for plain files is somewhat less perfect. Requiring users to reformat their drives in order to upgrade to Mac OS X is not exactly good business. The deciding factor may be the users.