It depends. There are a lot of parties involved before someone with a PD is treated in the face of the law. Of course if you are talking about something as heavy as murder or rape, there is full treatment. Think of it as a battle of competing experts, psychologists on both sides who will get to evaluate the offender and support their suggestions of the offender's propensity to reoffend with details about the reliability and validity of the measures they used in assessing the offender. Of course those psychologists will be under plenty of scrutiny themselves. Their credibility is gauged using questioning and analysis of their curriculum vitae, including all pertinent internships and supervision requirements.

Those guys are just one source of bias that can convince the decision makers (jury or judge) to treat the offender differently on the basis of the purported PD. The judge is supposed to be the fair and impartial lawyer in the courthouse. The other lawyers definitely aren't that way. Lawyers learn how to argue rather convincingly while earning their JD.

Many lawyers learn how to do voir dire (latin phrase, means to pick the jurors) while completing their JD. Sometimes, law firms even hire psychologists as jury consultants. Remember, the defendant chooses whether they want their case to be decided by judge or jury of peers.

Then again, you may not be talking about something like murder or rape. If you're talking about lower-level crimes, or you're not talking about crimes, there is a crazy amount of variance in the ways a PD can influence the way a person gets treated. One thing is for sure, diagnosis of histrionic personality disorder, narcissistic personality disorder, borderline personality disorder, or antisocial personality disorder does not provide evidence to support an offender's claim of the insanity defense. Those PDs also can't be the sole basis of one's involuntary commitment (unless they are a risk of harming someone). Parties in civil litigation might use this evidence in divorces, custody arrangements, probate cases, etc., if relevant, and lawyers are good.