I'm not going to get into the reasons why. Most of my readers will know why a high kick can be a defensive liability. But rather, I just wanted to comment on something I've experimented with while training.
I've lowered my heavy bag from the rafter enough to bring the height of both the knee and shin into play. This has taken the head height right out of the equation, as my heavy bag isn't tall enough to cover both. What this has done is twofold: my leg kicks have gotten much stronger, and I've been forced to strike in areas that exclude the area of the face (elbows and punches often inadvertantly gravitate to the height of the head while practicing).
Aside from the legs, the targets become the ribs, solar plexus, and neck area.
This works for me as it forces me into exploring alternatives to higher punches. I work more on knife hand strikes and palm heel more than I ever have.
Later, I'll raise the bag again in order to change the game once more.