But I wonder if she testifies what will come out of her loony mouth. She seems genuinely delusional.

Yes. It seems like it would be child's play to impeach her on cross - she's made so many public statements that are ridiculously false that any competent defense attorney should be able to tear her credibility to shreds.

I suspect that the value to prosecutors is less her testimony, and more as an example. That's a pretty light plea deal - no jail time or even house confinement, just probation. That's a good deal. And even though she may not be great on the stand, now that the DA has one high-profile cooperating witness they'll be able to get a lot of information about conversations with the other defendants that they might not have had access to before - so even if Powell is useless on the stand, she'll be a gold mine for discovery and trial prep. Those things should get some of the other defendants thinking about whether they want to try to get a no-jail plea.