The most recent CBA, the 2012-2016 Basic Agreement, has a provision that allows teams to add a 26th player to the active roster for scheduled day-night doubleheaders. The reasoning for this extra, temporary roster spot is obviously to help clubs get through 18 innings of scheduled baseball that would normally get played over two days. A short outing by a staring pitcher or an injury could be somewhat alleviated by the extra available player. After the second game is completed, a player must be sent down to get the active roster back to 25.
The Arizona Diamondbacks utilized this rule on Monday, May 27, 2013 when they had a day-night doubleheader scheduled with the Texas Rangers. They called up Tyler Skaggs, who pitched the first game of the doubleheader. Calling up a starter would make the most sense, as a good outing could eat up the bulk of the innings in one game, as Skaggs did, going 6 strong innings. (Even with the strong start, the Diamondbacks needed 4 relievers to finish the game, then had another 9 innings to play later).
Last season, the first with the new rule, the Astros utilized this rule and it was noted at the time that there is somewhat of a loophole in the rule. The player called up isn’t required to be the one sent down, and the player sent down after the doubleheader does not have to stay down for the normal 10-day period. So the Astros, in this case, sent down a pitcher that they knew wasn’t going to pitch for 5 more days due to the 5-man rotation, then called him back up when his turn came up. In the meantime, they kept the extra player they called up for the doubleheader, then sent him down 5 days later to make room for the callup.
Fair or not, this rule gives some flexibility to teams with scheduled day-night doubleheaders.