drop bear
Sr. Grandmaster
Your opponent's 45 degree downward flying side kick (with all his body weight behind it), aiming at your leading leg knee joint can be the most scary entering strategy that your opponent applies on you. Just to think that a 200 lb of weight is going to drop on your knee joint, it will put you in "defense" mode and that's for sure.
Which is I think the thing.
There is a reason why people don't shoot in on head kicks. And that is because the risk is generally too great. So a fly kick generally forces a defensive movement rather than a counter movement.
Forcing a defensive movement can give you a free opening. So that even if the kick does not pay off In a fight ending move. It pays off in a set up.
As a method of covering fifteen Meters and then engaging people it has its strengths.
You don't get much counter target. Eg I run in and punch. My head is exposed for a counter punch. I crib forwards in stance I am open to a blind side attack. I run that is a hard target to hit. And they have to side step and re enter to engage me.