Going with a six man rotation is a nice way to cut down on innings.
For what it is worth, when the Nats had a couple of opportunities to give an extra days rest to their rotation last home stand, they did not. They could have slotted in Detwiler the day after he was supposed to pitch in the minors and pushed back Zimmermann, Lannan and Martis a day but did not. I think they could have slotted in Stammen and pushed D-Crab from Thursday to Friday, IIRC, but did not. It seems they are trying to get the kids used to pitching every 5th day, so they may not want to spoil them with regular extra rest.
In terms of numbers, the 25th player can be a 13th pitcher (either 8 man bullpen or 6 man rotation), a 3d catcher, a 3d back up infielder, or a 6th outfielder. Figure the ordinary numbers are 5 OFs, 6 IFs, 2 catchers, 12 pitchers. If you have an IF/OF like Willie, you can go beyond the normal split.