Paul Randal of SQLskills.com does a great job of answering the question of why there are so many undocumented DBCC commands in his post Why are so many DBCC commands undocumented?
I would like to go on to say that not only does this apply to the DBCC commands but is some respect to all parts of SQL, other Servers, IDE's, Operating Systems, just about everywhere. There is always something that just does not make it into the official documentation.
And as Paul points out probably never will make it.
That could be why there are so many "Tips & Tricks" types of books, blog post, etc. everywhere you look.
And I also agree with Janos's comments on Paul's post, which was "I'm fine with them undocumented. All of us who need to use these commands know where to find "documentation" and whom to ask ".