The way I see it is that, right or wrong, Finn is in the Leia role from the first movie. He's the one with the mission to deliver the map (plans to the death star) and, although he assists in his own escape (rescue), he doesn't really do that much beyond that (action-wise). He's the one who gets BB8 to reveal the location of the base (just as Leia guided the heroes to the rebellion after her rescue) He also gets the 'stand up to the bad guy' scene as Leia did with Darth when he faces off with Phasma.
Poe is taking over for Han as the best pilot, and Rey? Rey is both Luke (Jedi) and Han (pilot/mechanic) simply to keep people guessing as to whose kid she is
However, this entire plot fails if it weren't for Finn. If he hadn't had the guts to leave, if he hadn't taken up where Poe left off chasing after BB-8 *and* warned Rey, if he hadn't delivered the plans *and* shown the Rebels the way into the Starkiller base...
He was critical to the entire plot, just as Leia was the first time around.
I loved Finn and can't wait to see him do more in the sequels. This time around, it was Rey and Poe's turn to be the Big Damn Heroes, but Finn was the driving force that kept the plot moving, and I'm sure he'll have his day!
I do hope they don't make the same mistake with Finn as they did with Leia, though, and have him 'sensitive' to the force and 'there is another' who never reaches their potential.