Hmm... if it was truly Undocumented and Verboten, MS would've made it an obscure dbcc call, an extended proc, etc. Instead, it's definition is there, in the clear.
It's useful for what it does, and at least the source is there for other enterprising people look at and "make it better". It's too bad there's not an effective feedback loop for some of those mods to make it back to MS and get into new versions of the product. ("$70 billion cash in the bank, and they say, 'we don't have the resources available to fix it'?")
Looking around other things in the database schema and SMO, it doesn't take too long to realize how many hacks are built into SSMS et al to work around things that are inherently broken when us mere mortals look at them, and that things in SQL Server in general work well enough without any glaring show-stopping bugs.