For the moment, her uses outweigh her liabilities. Orochimaru/Anko.
The seal on her shoulder isn't perfect, and sometimes it flares up at strange times; she will be practicing and suddenly black marks will crawl up her neck and down her arm. It's not perfect, but it's the best seal he's ever done and he's not prepared to call Anko a failure quite yet. Not as a new body, mind you, because he'd never intended to use her for that particular experiment. She has other uses.
Anko likes to think she's in control of the situation, legs still thinner than they are shapely splayed on either side of him and the cockiest grin a fifteen year old girl ever wore stretched across her face. She looks younger than she really is (shinobi training keeps them thinner longer, the young men and women in Hidden Leaf androgynous well past twelve and thirteen) but acts like she thinks she's five years older. And he lets her have her little-girl delusions of importance, because she never tries any of her little control games when other people are around and she can keep more than one kind of secret.
She's constantly in motion, and it's always decisive. There is never any hesitance or nervousness, not even the first time. It was fascinating, and part of the reason she had caught his eye for more than the usual liberties he took with his students. They always try to put on a brave shinobi face and he can always catch it for the lie it is, but she had never had anything but ferocity in her eyes. Even now her movements are those of a tiger shifting before making the kill, not the squirming of a girl unused to the position of power she sits in now.
"Good girl," he hisses, high praised indeed, but she doesn't react except to move faster and harder. It seems like things will be over soon enough with the pace pushed faster, but then her seal activates and black marks spill out onto her skin. She goes for his throat in that power-mad instant when the seal controls her rather than the other way around, a cobra strike that would have killed someone without jounin reflexes. He might even be proud to have taught her so well when he has time to think about it. Now, though, there is only time to lift her off and pin her to the floor in a way that isn't remotely erotic.
"You need to learn to control your power." Orochimaru waits to speak until the cursed marks finally fade from her skin, because it would be useless to tell her so before that. He has no use for those who fail at what he's set them to, and Anko knows it. "I didn't choose you to watch you play games like this."
"Then why did you?" Anko asks, her teenaged arrogance back almost before the curse has entirely receded. Jounin twice her age won't talk to Orochimaru like that, but her lack of respect is amusing enough to tolerate. Provided she doesn't try something like she just did again, that is.
"Sometimes I wonder." Orochimaru doesn't, really, because if he didn't know exactly why he kept her around she wouldn't be here. It wouldn't be anywhere near the first time he'd dealt with a useless student. But it wouldn't do to tell her that, now when she was already too cocky, too sure of her position in things. It would be better if he could find someone to put the other seal on, so that he could play the two off one another and keep control of them that way. But he hadn't found anyone else capable of holding a seal of that level who he could both trust in such a position and manipulate accordingly, and so Anko remained complacent and arrogant as the only one in his favor.
"Are you gonna let me finish what I started?" Anko demands, crude and brash as always. She does surprise him then, a sudden and perfect (he'd never taught her taijutsu like that, but obviously someone had) strike at his legs that sends him tumbling back to the floor under her. He's expecting her to try and pin him with her weight like she had before, so the fuuinjutsu is a second shock. She has her teacher's hands over his head and she's not even holding onto them, letting the sealing technique do the work for her. He had taught her that one.
"Very good," he manages, because it's true.
"Shut up," she orders. Her nails bite into his shoulders, too short to draw blood but just long enough to sting. "Shut up."
"Why?" He asks, smiling, pale lips parting for paler teeth, and she slams his shoulders down hard against the floor.
"Because I told you to." She snarls and then the seal on her shoulder starts to move again, serpentine and growing. Now her hands are off his shoulders and folding in a series of intricate gestures. He doesn't need to feel the heaviness in his throat to know she's managed to seal his voice, which isn't a hard sealing technique but it is one that isn't taught often anymore. She's been going through his scrolls, but that's hardly the important issue now. "You want me to control my power? Fine."
This situation is dangerous, even for him. There are a few options left for him, but all of them are likely to kill Anko and this isn't worth that. She seems to be in control of her own situation as well as his, so he lets her have her fun. It's his control game every other hour of every day, so let her have her few minutes of power. He smirks at her, serpentine almost in the literal sense of the word, and though her seal flickers it doesn't fade. It stays there the entire time, shifting with her movements as she directs everything. Is this how his students before Anko felt, trapped and silent? He hopes so. It's that thought that gets him off more than Anko herself does; they're too much alike, they both like being the one in control.
When it's over, she leaves him on the floor as she gets dressed again. With her shirt on again she can almost hide the fact her breasts aren't quite flat anymore; even hard training won't keep her thin and childish forever. If she's lucky, her arrogance and need for dominance will be overshadowed by her usefulness before she gets much older and it won't matter that Orochimaru will move on to newer and younger proteges.
"You want people to learn control, you have to let them practice it just like anything else," she says, because Anko is nothing if not convinced she is five years older and twenty years wiser than she really is. Then she makes the hand seals to release him and smiles as her seal fades, not her usual cocky grin but an even harder-edged smile that might as well be a sneer. "I bet that's why so many of your experiments fail. Nobody around you knows how to do anything but stand back and let you control things, and that's one thing they have to do for themselves."
She leaves as he gets up, rubbing at his wrists as the blood rushes back to them in pins and needles. He doesn't say a word to her as she walks up the stairs and back out to the street as if she doesn't have a cursed seal under her shirt and hasn't just put her teacher under two different sealing techniques and fucked him on the concrete floor of the basement where he does his forbidden studies.
No, not quite a failure yet, but she bears careful watching.