Recently, after Cole Hamels struggled in his last start, Phillies pitching coach Rich Dubee stated he is not ruling out the possibility of a six-man rotation. This would be a mistake.
It seems Dubee, like many fans, have become results orientated with Kyle Kendrick. Once again, some fans are thinking Kendrick's current 3.25 ERA means he's a good pitcher. They see a low ERA and assume a pitcher is better than what he is. Kendrick currently is a prime example.
He's still pretty much the same pitcher he's always been. While his year-to-year ERA is very volatile and much more susceptible to variance and luck, looking at more skill-based metrics are not. When looking at his peripherals, nothing really has changed.
While he has decent control and can get some grounders, he still cannot miss bats. Even though his 4.45 SIERA is essentially at a career best (not counting 2009's 26 innings), he's still pretty much the same pitcher across the board.
While normal luck stats, such as BABIP and left-on-base percentage, have been a tad bit luckier than his career norms, it does not exactly jump off the page. Having to look deeper, the main culprit in his much lower ERA is what's been occurring with runners on base and in scoring position, where his BABIP is an unsustainable .214 and .212, respectively.
Not only is Kendrick not the pitcher his ERA dictates, the use of a strict six-man rotation in itself is not optimal. Pitchers, on average, typically do worse on six days of rest than on four or five days of rest. In "The Book: Playing the Percentages of Baseball" they performed a study comparing starting pitchers in regard to how many days of rest they received:
"There were 4,456 starts where a pitcher had exactly five days of rest. Their wOBA in these starts was .346. On four days of rest, the same pitchers had a wOBA of .350."
As for six days of rest:
"We have 645 games in those cases, with a wOBA of .355."
Keep in mind that most of the time in a six-man rotation, pitchers will get six days of rest and not five, because of the typical off-days on a Monday or Thursday.
Even if there were no off-days and every pitcher got exactly five days of rest each time, the slight increase in overall performance would most likely be more than canceled out by the fact you are sacrificing starts for your best (Halladay, Hamels, Lee) in favor of getting starts for your worst (Kendrick).
In addition, the Phillies cannot afford to go without a long reliever in the bullpen. The last thing they need is to use Lidge, Madson and Bastardo for multiple innings if another 19th inning game arises, or a starter needs to be pulled early. If they want to make a spot start for Kendrick every now and then when days off aren't on the near horizon, then so be it, but do not make the mistake of making it an every week occurrence.