The short answer is discovery.
While I understand the logic of joining stepparents as parties to custody or visitation proceedings when that stepparent will not behave around the child(ren), I remain convinced it is bad strategy. Not only does it double the number of adversarial parties, it allows the stepparent to participate in all the proceedings. I have often proven an opposing party is lying when that party and his or her spouse testified inconstantly on important issues at trial. This is much less likely to happen when the stepparent is in the courtroom during the parent’s testimony. Further the stepparent or his or her attorney is allowed to issue discovery to my client and to question my client and witnesses at trial. Finally, when a stepparent is misbehaving, my goal is to get the parent to control his or her spouse’s behavior or suffer consequences for failing to do so. Giving the court the ability to issue remedies against a stepparent takes the focus off of the opposing parent.
However there are times when a stepparent needs to be joined as a party to force the stepparent to submit to discovery. Often one will want to get a stepparent evaluated for conditions that might make it unsafe to leave children in his or her care or home, such as substance abuse or serious mental illness. An unfit stepparent living in a parent’s household can be a basis to change custody or limit that parent’s visitation. That stepparent will need to be joined as a party before being ordered to undergo testing or evaluation.
Other times, the stepparent’s financial information might be relevant–especially when it is alleged that the parent is shielding income or assets through his or her spouse. The right to issue records subpoenas to non-parties and depose a stepparent, once-and-only-once, can uncover much of this information. However, requests for production are broader than subpoenas, and one can also issue interrogatories and requests for admissions to parties. Subpoenas and a one-time deposition may leave much information uncovered.
Join a stepparent as an opposing party if it is essential for discovery. In over twenty years of family law practice, I’ve yet to determine any other good reason for doing so.
I worry about joining parties who may not be necessary. When I was younger, we brought a lot of suits for malicious prosecution and abuse of process. Joining a party to make him or her subject to discovery seems to be using a process for other than its intended purpose. Scary.
On the other hand, the form for the UCCJEA affidavit suggests if there is a person who claims a right to custody or visitation, he or she may be a necessary party. This might justify making him or her a party defendant.
I may name others as parties but it will be only after a lot of legal research.