a) She has intelligence regarding Death Eater training, customs and future plans that we can utilize. She is also a human barometer with the Dark Mark, at least able to tell us where they are summoned to.
b) She's given us the @deatheaters hooter account and that data is prime for mining.
c) She can provide a sworn statement as a former Death Eater and decry their aims, their training, their methods and help us influence outcomes.
d) Surrendering would be proof that we aren't monsters. That, by keeping and incarcerating a Death Eater until we prise the government back from their control, we care about equal justice and the rule of law.