Plutarch is the Capitol's former Head Gamemaker who's now on the rebels' side of the war. Yet he's still acting in much the same kind of position he held when he was on the Capitol's side. The only difference might be that he has more power, to an extent, as a rebel – a higher position of authority. That said, he's had to make sacrifices and give up the kind of special treatment he had in the Capitol in order to further the rebels' cause. Ultimately, though, he's still the ringmaster, the media manipulator, and the guy responsible for many of the actions the rebels take.
He's always thinking about presentation, as Katniss explains when she asks him about why Peeta is getting to train with the rebel soldiers:
When I confront Plutarch, he assures me that it's all for the camera. They've got footage of Annie getting married and Johanna hitting targets, but all of Panem is wondering about Peeta. They need to see he's fighting for the rebels, not for Snow. And maybe if they could just get a couple of shots of the two of us, not kissing necessarily, just looking happy to be back together. (18.4)
Plutarch doesn't seem to care as much about how Peeta and Katniss are really doing as he does as how they appear "for the camera." He's got to keep the celebrity versions of Peeta, Katniss, and the other victors in mind all the time in order to present the best possible media versions of them to the rebelling districts.
Plutarch is the mastermind behind many of horrifying trials Katniss, Peeta, and the other tributes had to endure in the Hunger Games arenas. He contributed to their suffering, and so it's strange that he's now helping them get out of it. For example, when Plutarch presents the rebels with the Holographic design of the Capitol, he says:
Each light is called a pod. It represents a different obstacle, the nature of which could be anything from a bomb to a band of mutts. Make no mistake, whatever it contains is designed to either trap or kill you. […] To be honest, I created a fair number myself. (18.13)
The funny thing is, Plutarch designed a lot of the traps the rebels will face within the Capitol too. He doesn't know them all, but he knows some; therefore, he's helping the rebels prepare for and plan around the nearly impossible traps he set years ago. They need him to help them fight the war, but in his past life he helped their opponents prepare for the very same battles. In a way, he's trying to out-strategize his old self.