As for Finn's lack of importance in this movie, it is really frustrating, but I think it's so important that they fail as often as they succeed. The Finn/Rose/Poe mission not only fails, but results in the deaths of dozens of Resistance soldiers when the transporter ships are gunned down. And that's war. Finn and Rose wouldn't have been deemed useless had the plan worked. But they were naive and desperate, so they made mistakes. And as a moviegoer, it's frustrating to spend so much time with them and have no payoff, but that's what made TLJ so hard to watch, but - ironically - rewarding. That it is so devastating for so much of the film. It just made it all the more powerful and engaging for me.
That being said, yeah, Rose felt kinda forced and may very well end up being completely useless.