The prosecution called NBC and OW ( they would have had a subpoena to attend) because they were first on the scene so to speak therefore it is in the interests of fairness of presenting the case that they got to have their evidence on record. Even if this means that some of what they say may not help the prosecution's case. I also think that they were called early so that their evidence that supports the defence is out there early in the case and there is evidence that later refutes it. I think OW's evidence that I was in court for, went into minute detail about past depression but when asked if she had any concerns about Allison's mood in that week, OW said no. So the prosecution gave her lots of latitude to ramble on about depression and all the help she gave her. That was good strategy in my opinion as it got it out there then made it simply not relevant to Allison's current mental state. I call it the taking the wind out of the sails strategy.