Defences can be effective because they ensure that people aren't wrongly convicted for their actions. Some may not have been able to control their actions (e.g. involuntary behaviour or automation, duress, diminished responsibility) or they may not have had the criminal intent necessary for them to be convicted of the offence (e.g. mental illness or insanity, involuntary behaviour/automation). The use of these defences protects the individual's rights and applies the rule of law, as well as making our justice system more efficient since we aren't throwing every single person accused of murder into a prison.
As well as this, if someone has used mental illness or insanity as a defence, they may be given resources to assist in rehabilitation (I'm not too sure about this one, probably better to look it up just to double check).
A case was in the news recently where a woman was recently acquitted of the murder of her infant on the grounds of it being infanticide due to her being depressed. Can't remember what the case is called but you could probably use it as evidence. Of course you could also discuss the negatives to this case, as she was left unpunished and the family is left without justice.
I hope I helped
