Pitching is an art form, but not every pitcher in Major League Baseball paints the same way.
Some use a wide array of pitches that they can control effectively, thus making it harder for hitters to anticipate what's coming from pitch to pitch and from at-bat to at-bat. Others prefer to keep it much simpler, sticking only to three, two or even just to one pitch.
Some of those guys get by OK. Mariano Rivera, for example, has made himself the greatest closer of all-time with his cut fastball. R.A. Dickey doesn't need much to succeed other than his good-luck-hitting-me knuckleball.
They're the exceptions to the rule, though. Variety is better when it comes to pitchers, and there are at least 10 pitchers out there who need to develop some variety in order to either keep their star status or achieve it for the first time.
Let's take a gander.
Note: Stats courtesy of FanGraphs.