In the years since McCracken's discovery, some caveats have been added to his theory. It was found that knuckleballers have more of an effect on balls in play than do other pitchers. And pitchers do, as previously believed, have certain tendencies toward allowing more groundballs than flyballs, or vice-versa. However, whether you give up groundballs or flyballs, you still can't control where they're hit — the rules still apply.
This knowledge was a great help in assessing which pitchers are really good and which ones are lucky. Pitchers who have a very low ERA, despite poor walk, strikeout and home run numbers, are almost guaranteed to see their luck run out. No one knows when; their luck may last for a season or just for a few weeks. But it will end. Many pitchers who "take a step forward" actually just get lucky one season, much to the chagrin of the team that drops a big free agent deal in their lap.
Another bit of baseball wisdom confirmed by McCracken's theory was that the best pitchers are those who get strikeouts. Limiting walks and homers helps as well. If you want to see a list of the greatest pitchers in the league at any given moment, take a look at the strikeout leaderboard; it's the most informative simple pitching statistic out there (better than wins and even ERA).
Now a lot of people think this theory is hogwash. They will start naming lots of people who succeeded with control and command rather than strikeouts — and Greg Maddux is usually at the top of the list. Maddux wasn't a strikeout king, was he? No, but he struck out more people than anyone remembers. Maddux topped 200 strikeouts once (1998) and struck out more than 150 batters in eleven seasons. People have a hard time accepting this argument, but if you've got the Baseball Encyclopedia with you, you'll win the discussion in no time.
Granted, you don't have to strike out 300 guys a year like Randy Johnson to be effective; but it's almost impossible to succeed as a pitcher with a below-average strikeout rate. Look at all of the great pitchers in history; even if they didn't lead the league in K's, they almost definitely struck out more than the league average.