Stretching prevents injuries, blindsided.
Stretching wouldn't do anything for you however, if your as mobile as a log. Then you wouldn't need to be the least bit flexible!
As far as kicking cold and Mr Kurz, well that is really just to prove that his method works. As far as him using high kicks for self defense, I don't know. Hell I don't even know if he is a Martial Artist!
There are plenty of proven methods of stretching, but not all of them are as safe as the method(s) Thomas Kurz outlines in his book.
I do agree that high kicking is risky as hell in self defense. I wouldn't kick above my waist myself, if at all, EXCEPT in training.
High kicks are fun and a great way to stay in shape, but thats just my opinion.