I'm unable to address directly the question of the Information Commissioner's position, but I can say quite affirmatively that departments do use other categories. The difficulty may be in a question of interpretation.
As I understand it, Treasury Board policy is that every department should use that limited number of basic categories. In the annual reports regarding the operation of the Access to Information Act, that limited number of categories is used. But as I have said, based on my research published in 2005 and earlier, it is clearly the case that departments use other categories, and the one category that is used by every other department that I have examined essentially captures political requests. The label may be “member of Parliament” or “political party” or “Parliament”.
Indeed, if you look at the monthly reports of incoming requests that are maintained by an organization called Open Government Canada—I just checked the August 2006 report before I came in to this session—you can see requests identified as coming from Parliament. It's unambiguously a current practice.