I heard an interesting commentary by Dan Abrams, I believe. He said if this weren't a death penalty case, absolutely she should take the stand for the Hail Mary because the defense case has completely fallen apart, and there's nothing to lose.
However, because it's a death penalty case, there's a risk of her taking the stand and the jury thinking she's lying. Then, when she is convicted, the jury won't believe anything she says in the penalty phase -- they won't have any compassion or give any weight to any mitigating testimony she could provide.