All numbers presented herein are based off 2009 American League totals, but I do not expect them to differ much in previous years or in the National League.
Pitchers with high ground ball rates are good. Like Felix! Felix is good. You should prefer ground balls to fly balls. Fly balls are bad. The standard rationale goes like this: home runs allowed per fly ball is a stat that does not appear to be under much control by the pitcher and since ground balls never leave the yard, the more balls you keep out of the air, the fewer home runs you allow. Not allowing home runs is good.
Some people counter that argument with the fact that ground balls go for a hit more often than fly balls. While true, the difference between the two is vastly overstated. Strictly between ground balls and fly balls (excluding bunts, home runs, line drives and infield flies) ground balls go for a hit about 3% more often. That is not much.
It is even less once you factor in the salient facts that ground balls become double plays far more often, non ground-balls see a higher slugging percentage allowed on average and that excluding line drives is a misleading distinction. You can read this all and just believe it, but how about some handy numbers to give you some nice foundation support when you go to convince others?
The average ground ball (including bunts) generated 0.04 runs and 0.80 outs
The average non-ground ball (fly balls, line drives, pop ups) generated 0.23 runs and 0.62 outs
On a runs per out basis, balls in the air generate almost 7.5 times as much offense as balls on the ground do.
Non-ground balls were:
A fly ball 49.3% of the time
A line drive 31.5% of the time
A pop up 12.5% of the time
A home run 6.7% of the time
Ground balls were:
A ground ball 96.3% of the time
A bunt 3.7% of the time
Out factors were derived from average number of outs recorded per batted ball type. Run factors were derived from changes in score and the run expectancy matrix after each play per batted ball type. Errors were factored in as well.