2011 has seen an invasion of at least occasional 6-man rotations, with the Rays, White Sox, Yankees, and Royals all trying it at one point or another. D-Rays Bay has a nice article about it, including some past research by Tom Tango and colleagues in The Book. Here's an excerpt:
On 3 days rest (113 starts) pitchers had a .369 wOBA against and the same group of pitchers had a wOBA of .352 on 4 days rest.
There were 4,456 starts where the pitcher had exactly 5 days of rest and the wOBA against was .346. On 4 days of rest the same pitchers had a wOBA against of .350.
There were 645 games where the pitchers had exactly 6 days of rest and the wOBA against was .355. On 5 days of rest the same pitchers had a wOBA against of .346.
Got that? Said again another way, it's:
Going from 3 to 4 days rest drops wOBA .17 points.
Going from 4 to 5 days rest drops wOBA .04 points.
Going from 5 to 6 days rest raises wOBA .09 points.
(a lower wOBA against is better for a pitcher...means a lower OPS against, just measured better)
So you get a large advantage by going from 3 to 4 days rest (that's a typical 5-man rotation, four days off per pitcher), and a very small additional advantage by going to 5 days rest (that'd be a 6-man rotation). And going to 6 days rest you have a drop-off, probably because pitchers are rusty.
Now, the extra advantage of going to five days rest is probably statistically significant, but it's a very small advantage in terms of actual meaning in baseball games. There may be additional benefits, however, in terms of reduced wear and tear at a season level on the top tier of starters, that are not measured here and might be really important. But for most teams, it probably isn't worth it to take starts away from your #1 or #2 starters and give them to your #5 and #6 starter (the #5 gets skipped far less in this scenario than if you do a four-days-rest rotation).
In the Reds' case, however, I think it might be a good idea. The reason is that the Reds' staff is basically a bunch of #3's, #4's, and #5's. And Travis Wood and Edinson Volquez, major league pitchers, are in AAA. So there's really not much of drop-off between the Reds' top starters and the guy who would be the #6.
Just for curiosity's sake, I went through the Reds' 2011 schedule and took a look at what a five day rest rotation (six man rotation, skipping back-end starters when you have off-days) would look like. Here's the distribution I got:
#1 starter: 29 starts
#2 starter: 29 starts
#3 starter: 28 starts
#4 starter: 27 starts
#5 starter: 26 starts
#6 starter: 23 starts
So we're looking at around 3 or 4 fewer starts by each of Cueto, Leake, and I dunno, who's the #3 starter...Willis? And you end up with those guys being more rested, and probably at lower risk of injury. Mike Leake, for example, is just 4 IP shy of his 2010 total, and we all remember how he ran out of gas last year. Maybe this would help keep his innings under control? The cost is that you get more starts by either Travis Wood or Edinson Volquez, who have 2011 MLB xFIPs of 4.47 and 4.10, respectively (i.e. really, are they that much worse?).
At this point, the season is a lost cause anyway, so doesn't it make sense to give Wood and/or Volquez some additional major league starts to see if they can get back on track? And at the same time perhaps reduce the risk of injury to the guys that are getting it done? Thoughts?