Not to mention, K/9 misses an important factor: how many hitters did that pitcher face.
Hypothetically, a pitcher could throw an inning facing a lineup three times, K'ing the 9 hole hitter each time. His K/9 is a gaudy 27, but he gave up 24 runs in that inning.
Conversely, a guy could get three straight pop-ups and have a K/9 of zero, but he had a 1-2-3 inning. Or he K's the first 3 hitters, and has a K/9 of 27...same as the first guy above.
Not a very useful stat.
Those are extreme examples, of course, and over time K/9 becomes more useful, but K percentage is always a better stat.