During the time of STATE OF DIVINITY, the Qwai Fa Bo Deen was *the* coveted martial art of wulin. To rule the wulin of SOD, mastering the Qwai Fa Bo Deen was the way to go.
Of course, the manual had a very significant design flaw; to practice it, its practitioners (who must be male) must sacrifice their masculinity.
If you ask me, I'd say Qwai Fa Bo Deen was the preferred martial art of its time because it was the *only* elite martial art of its time. It has never been proven that the Qwai Fa Bo Deen was superior to 1 Yeung Finger/6 Mak Divine Swords, Yik Gun Ging, Bak Ming Sun Gung, Dook Goo Kau Bai's sword arts, Hong Lung 18 Palms, 9 Yum Jen Ging, 9 Yeung Jen Ging, Ha Mo Gung, Lung Jeung Bor Yeh Gung, Sad Palms, Keen Kwun Dai Loi Yee, etc. Mastering any of these arts would make one a wulin elite...and at a MUCH lesser cost than practicing the Qwai Fa Bo Deen.
Consider this: you're an ambitious wulin man. On a table in front of you are the manuals for 1 Yeung Finger/6 Mak Divine Swords, Yik Gung Ging, Hong Lung 18 Palms, 9 Yum Jen Ging, 9 Yeung Jen Ging, Keen Kwun Dai Loi Yee, and the Qwai Fa Bo Deen. Who in his right mind would choose the Qwai Fa Bo Deen?
This "great" martial art might be powerful, but its severe design flaw makes it the least desirable of the elite martial arts; it was only coveted in its time because it was the only one left of its kind.