It does give Rowan power over them... eventually. Whether they're aware of it or not the more they rely on Rowan to build their ability to conquer the world, the more he's able to work on his own agenda, the more capable he becomes of rebelling. If we're going to continue using Dark Knight Rises as a comparison point, Rowan is Batman. He has been knocked down, he has been beaten, he has been broken, but he has the skills, knowledge, and will to rise up, take whatever advantage he can, and eventually defeat the twins. You're quite literally comparing him to someone that can be gotten rid of at any time, and he is, but losing him would set the twins back years. Rowan is the difference between a relatively quick, straight forward, comparatively bloodless campaign to years of pitched wars that the twins would likely lose. If the odds weren't so stacked against him Rowan would be a Gary Stu since he's handsome, a skilled orator, a natural diplomat, a seasoned general and an extremely skilled warrior with an expertise in multiple forms of combat and has shown himself to be so skilled that in a proper fight Andras would stand as much a chance of losing as winning, oh, and he's got that good dick. You're selling Rowan far, far shorter than Seeds of Chaos presents him as.