John Gruber makes all the right points (http://daringfireball.net/2009/10/congrtlns-osx) but fails to mention one crucial one: There never was an (official) interface to set creator codes. He correctly concludes that:
“The creator code long ago stopped meaning “the app that created the file”, and instead meant “the app this file should open with by default”.
If people had used the creator code to mark individual files as destined for one application and if this ability were now gone, there would be reason to complain. But no application I have ever seen had a check box: ‘Set creator code’ and no part of the Finder allowed you ever to set the creator code. Only third-party utilities did let you do this and and required that you knew the precise set of characters. Hardly different from how the situation is now.
To follow Gruber’s own reasoning, if the preferred application for any given file nowadays is more likely than not to be an application other than the one that created the file, surely, creator codes are more likely to result in an unintended behaviour than not. Particularly, given that the user has no choice whether an application sets the creator code or not except for a file-by-file manual override later. There are good reasons why basically no application still sets the creator code (if you subtract those using a custom file name extension, making the creator code already pointless).