I think one would be the idea of saying that the information should be relevant to the purposes and principles of sentencing for the accused; that it has to apply to the individual's case. Also, as I indicated I think the individual has to consent to having that information brought forward. There are still lots of reasons they would not want it brought forward.
For us as a society that cares about just and effective sentencing, it is important that the sentence be fit for the individual circumstances. The extent to which people would rather take a tougher penalty because they don't want their mental health status to be factored in is a bit of a challenge.