If the SO is welcome, I'd bring the person--repeat if the SO is welcome. I would not bring them if the person was not welcome. If you don't know ask ahead of time.
For PDs, Chief Residents and program coordinators, sometimes extra people can create a logistics problem in terms of budgets, seats needed at an event etc. It could look rude if you brought someone in uninvited.
If the person was invited, I'd say bring them in. While I was chief, while we never had a dinner, if we did, I would've loved to have seen the SO. The decision to go to a program is often highly influenced by the SO, so for that person to see the program and why that program is on their partner's mind could be beneficial for the program and the candidate. It also can help to break the ice in getting to know the candidate, and vice versa-for the candidate to get to know the program.
Most people do not factor that relationshpis can have an large effect in a person's choice to pick a program. I know it certainly did in my case. My wife did not want to be in NYC which nixed Albert Einstein which I heard from several gave the best teaching experience amongst the forensic fellowships in NYC. She did not want to live in New Orleans which nixed Tulane off my list of choices, and both programs accepted me, and I strongly considered them. Thankfully, I still did go to my #1 choice, but choosing between those three went from difficult to very easy once she told me her feelings.