I have wondered about this aswell. the only kata I know of that has the jump kicks in them are Kosokun dai, Chinto, Superempi, I have seen the shotokan unsu that has the jumpspin cresent but in the Shito ryu version that does not apper. I useally bunkai these as either 2 front kicks or it works really well if you have some one charging in on you and do the jump front kick as soon as you come up it with your knee to get up it stops them. othere then that I dont see much use for them.