This is a rather unusual situation in which a defendant took the stand (twice) to give testimony that essentially constitutes perjury if she now cooperates. Ordinarily, this would so damage a defendant's credibility that her testimony would be of little value (the trusty "are you lying now or were you lying then" cross examination is devastating). However, this is an unusual case where there doesn't seem to be ANY other credible theory for the murder. In most cases there are stray claims that a drug deal went wrong (see Ron Goldman / Nicole Brown Simpson murders) or a burglary gone wrong etc. In this case we have convicted hit men in a crime where the Adelsons have the obvious motive and the defense really is going to have to stretch to offer any sort of alternative theory. Given how powerful the circumstantial evidence is, I can see where it might be quite valuable to have Katie testify even if she doesn't have anything new to add other than "yep, we all did it and we did it for the Adelsons." Even though the jury would obviously look at her with contempt, hearing such testimony out of her mouth would be dramatic and powerful.
When I used to practice as a trial lawyer we were always mindful that our primary role in court was to be a storyteller. The jury has to be engaged and persuaded; and calling a witness like Katie is almost a no-risk proposition given the lack of a credible alternative theory in the case. Indeed, unlike Luis Rivera, Katie was actually "in the room where it happened" and will be able to describe chilling details that we've never heard. Sure, the defense will savage her and destroy her on cross, but you can't really get hurt falling out a first floor window. I think Georgia and crew are planning chess movies using Katie to tell the story of what happened here and they really don't need to worry about her credibility because Charlie sure isn't going to take the stand and her testimony will put enormous pressure on the defense.