My Name is Mud
/I was taught an additional reason for putting the tongue on the roof of the mouth: In meditation/stillness, it allows saliva to pool and then descend down one's throat with out creating the gag reflex or having to actively swallow. Not particularly useful for pure martial arts though.
Perhaps because of my California coastal experiences with mud I think of baguazhang mud walking happening in sticky mud, instead of the slippery or calf-deep mud he describes. The advantage of sticky mud is that it emphasizes the opening and closing (kaihe) of the joints while giving the same emphasis to the back foot that calf-deep mud would.
Of course, if you have been practicing with the slippery mud idea that has the advantage that you don't rely on a fixed root; a higher level practice actually. I suppose the next level up might be walking on top of quicksand, or really high level--water.