In a lot of discussions about AD&D (1e, for you purists out there!), a repeated concept is the idea of "secret rules". Things players were not meant to know. Whether it's your precise chance to hit (at least before the publishing of the Player's Screen in '85), or not telling your Assassin that he can train to become a master of brewing poisons, there seems to be this clear line of demarcation between what players should know, and what DM's should know, as evidenced by Gary's comment (presumably in jest?) about any player reading the DMG is deserving of a less than noble death.
Not going to get into the philosophy of that here, but what I find myself asking is, were DM's never supposed to play the game?
Because the instant one learns how to be a DM, by reading the DMG and running adventures, they now know all this stuff. Was it just assumed that anyone who had this information would simply pretend not to, sort of a gentleman's agreement, or was it assumed that all DM's would be "forever DM's", and once you "graduated", that was that?
I highly doubt the latter would be the case, since Gary himself played the game!