In over fifteen years of playing and DMing I can't recall ever facing or running one. There are many "classic" D&D monsters for which I'm at a loss to understand their position as "fan favorites". Sometimes I suspect that the game designers just keep updating them lovingly from one edition to the next because they themselves remember them fondly; but which the rest of us simply consider one more random (and oftentimes uninteresting) monster from the manual. Likewise I don't understand the fascination with beholders, githzerai or githyanki. :\