I've found it to be inferior to the other less common drop. The sacrifice you have to make for her rare gun is too much for much smaller gain than what you can accomplish with her default and uncommon. Her rare seems to be the most inconsistent and, at best, situational gun out of all of them. It was a real let down when I won it only to find out the other 2 were better. I keep going back to it thinking there is something I'm not getting, but nope, it sucks. The default does what it's trying to do a lot better. The increased damage comes at a silly cost and the gun seems very quirky.
I can't think of the situation in which it would be netter than the other two. Certainly not close combat, and certainly not ranged. The parasite siphon distance is abysmally smaller than the other two, you practically have to be kissing them for it to be active. The duration is also horribly shorter than the other two guns. It only does damage when you rreach a threshold, but by then you have no ammo left to shoot, and even then it no longer siphons.