A common theme in the discussion is "Why not, what do you lose with extra directions?"  

At Lands of Aethar we created a status ailment called "disorientation".  When a player is disoriented, they lose the ability to tell direction, and see things in terms of "forward", "backward", "left", "right".  Unfortunately, we're a SMAUG MUD with the diagonal directions (they were installed years before the disorientation code was).  Diagonal direction makes disorientation more awkward than I'd like.  If the disoriented player is facing north, they will see a northeast exit as "forwardright".  Rather kludgy.

But, then again, LoA is the only MUD with disorientation, in part because a feature like that requires sweeping surgical code changes to the deep core of the codebase, whereas the vast majority of "coders" know how to make minor text changes and not much else!

But on a "cookie cutter" MUD without advanced features, from a pure builder perspective, I'm very fond of diagonal directions.