If she's going to throw her life away, the least we can do is put it to some use.

Besides, if she gets her spirit powers back, she'll be able to help Setsu fight against any threats.

Setsu, while skilled, isn't all that powerful alone.

Besides. This way she's easy to keep out of our hair. Not to mention it's almost an eye for an eye as far as what she's done to us.

...and if you really just want her gone, you can order her to kill herself.