After The Trade last offseason, Dayton Moore and Ned Yost began preaching the importance of the royals getting 1000 innings from Kansas City's rotation, claiming this is an important benchmark to reach when competing for a playoff spot. The Royals came up 13.1 innings short last season, and Dayton Moore has not been as outspoken about reaching this benchmark in 2014, but for better or for worse, the idea continues to linger.
The Vargas signing is evidence of Moore's pursuit of starter innings. Vargas has thrown 200 innings twice in his career, and is one out shy of 100 innings just 69 games into 2014. When he appears on the IP leaderboard, Vargas is usually accompanied by players with much more name recognition like Hernandez, Kershaw, and Greinke. In 2012, Vargas was 10th in starter innings, but had a significantly worse ERA or FIP than most of the other players in the top 20, so Vargas is clearly attractive to a GM that values innings pitched without having to pay for ERA.
It's clear that "innings eaters" are a key part of Dayton Moore's roster construction strategy, but what value does this actually add to a major league team? Starter IP and team ER do follow each other very closely, with more innings pitched closely correlated to fewer earned runs.
It would be nice if this necessarily meant that "innings eaters" made a team's overall defense better, but this does not appear to be the case. Instead, the causation is likely to be the other way around – starter IP correlates even more closely to starter ERA – so in reality the best pitchers stay in games longer because they get more batters out and the manager feels no pressure to pull them (or feels pressure to keep them in).
There is an argument that getting more innings out of your starters would lead to better bullpen performance. Because major league managers tend to use their best relievers in late innings, starters that go deeper into games would get the ball in the hands of a team's best relievers without having to go through weaker middle relievers. This is not the case, however. Although more starter innings do correlate with a slightly smaller bullpen ERA, the signal-to-noise ratio is much too high to draw any meaningful conclusions. And even if this was a perfect causation relationship, the results suggest that an additional 100 starter innings would only lead to a 0.3 difference in bullpen ERA, so starter IP really has no appreciable effect on bullpen performance.
What does this mean for the Royals? As Craig Brown pointed out last year, reaching 1000 innings is not a magical threshold that guarantees teams playoff spots, and it is very possible for a rotation to throw a lot of innings and still put up a bad ERA. Fortunately, Ned Yost doesn't seem to be keeping starters in just for the sake of keeping them in this season, and in fact he seems to have been somewhat conservative with use of Ventura and Duffy so far. Starter innings are a result of quality pitching, and not the other way around, so if Kansas City's starters reach 1000 IP it should be celebrated as an indicator of good performance. If this threshold is reached without a good ERA or FIP to support it, then 1000 starter innings is absolutely meaningless, and will not appreciably overflow into any other aspect of the team.