Some companions will leave the party permanently if you do not bring them with you for their subplots. It’s not metagaming to put them in the party for things that are relevant to them, it’s the opposite, it’s bringing them along for something incredibly important to their personal journey.
The game I’ve laughed the most while playing is Jackbox. Fibbage and Quiplash have me in stitches every time, although it depends on how funny your friends are.
Trading specifically I’m not sure if it changes anything, but you can buy things from NPCs that normally you can’t trade with. I do know that stealing from NPCs and attacking before a cutscene plays out can change the survival of some NPCs.
For the fight in Grymforge you’re talking about, there’s also an opportunity to get some of the duergar on your side, which makes the fight a lot more manageable.
Also, the game never explains this, but you know the buttons in the bottom left of the screen during cutscenes? You can use these to trade with characters that don’t normally have an option for it, to switch party members to sneak around while the one in conversation distracts an NPC, or to attack before a cutscene finishes playing out. Doing these things can affect the outcome of game events!