The strikeout must truly be a pitcher's best friend. Not to say a pitcher cannot succeed without striking out batters, but boy does it seem to be a big help.
Taking a look at last year's K/9 leaders, there were 30 pitchers with at least 140 IP and 8.0 K/9. Of those, 24 had an ERA below 4.00. Also interesting, I thought, was the fact 21 of the 30 gave up fewer than 1 HR per 9 IP. Of the six pitchers with an ERA above 4.00, five gave up more than 1 HR per 9.
Conversely, out of the 30 pitchers with at least 140 IP and the lowest K/9 rates, 20 had an ERA above 4.00 (and 6 were north of 5.00) and 15 gave up more than 1 HR per 9. Of the 14 pitchers with a K/9 below five, 12 had an ERA above 4.00.