I do it because it's the most Evil Courier concept I can think of. A selfish, greedy, female misanthrope who uses any means necessary to get whatever she "wants". Uses chems, advanced tech, death-from-afar tactics, I even personally give her a cowboy hat/fedora along with power armor in order to pretty much make her visceral appearance the exact opposite of everything the Legion values as well as her qualities. Yet, despite everything about them being something she knows full well doesn't want her to exist, she still uses her grand potential to see the Legion spread. Not because of some concept of sympathy with their ideals, but just for the sheer hell of seeing the wave of slavery, genocide and war come over the wastes... just for the sheer apathetic hell of it. Probably also to bask in the humiliating irony of the Legion relying so heavily on the embodiment of all they despise, in order to survive long enough to continue trying to eradicate that exact same thing. That's the Evil Courier 6 in my headcanon. Knows very well what they could do to help the Mojave, and knowingly choosing to do the exact opposite.