@radiofr33 One place I see NPCs as useful is if a player wants to tackle some non end game content alone, like daily expedition. They do contribute dps and heals/cures in those specific situations. That way you can do your 5/5 daily quickly and not having to wait for other players for it. I always pick NPCs who can heal, like summoners/forces/techers/bouncers as their main class.
A few perks about NPCs. Opening your Quick Menu (Pressing select on xb1 controller) and selecting "Instructions for allies" opens up a menu that lets you change two things about NPCs. You can have them stop attacking if you need to, like for a capture quick trial for example (they will still heal/cure and buff though). You can also choose if you want them freely using their PB or wait for you to release yours for a chain. When battling, you need to be close to the enemies yourself as NPCs prefer to stick close to you. So if you run far from enemies, they will run with you. Of course, when selecting who to bring with you, I always prefer to use the 'Free' option, as usually NPCs made by players may be much stronger than story NPCs. Or if you have a good aux or a geared friend avatar, choose that as well.
From what I've seen about ult content though, they fall somewhat short of it so I can't recommend them for that content. I'm actually surprised those rooms even have them as an option.