my oppinion is that bouncing is extremely importartant, as poeple here have said they make the opponent less aware of your movements and such, i personally doesnt bounce around so much due to the fact that im a bit overweigh , but, by just keeping my heels of the floor and shifting weight between my legs i stay fast and agile, so when i meet someone in a competition for example they dont see me as a fast moving opponent, but after a few surprising kicks to the head they learn
and when im pushed back so i have to plant my heels i usually try to give a fast pushing kick to their thighs or stomach that will give me time to reposition.
though if you are a light person i imagine "stance fighting" could be usefull in some occasions, but from my experience i'd say its always best to keep bouncing just to have alert legs
/björn