Except the plot of the Mass Effect games have always revolved around humanity.
The thing about the first Mass Effect game is that the whole "Humanity is in danger" schtick was simply a coincidence to a much larger scale plan. Saren didn't attack the human colony because it had humans on it, he attacked it because it had the Prothean beacon on it. The big plan was targeting every sentient race in the galaxy.
I think when it comes to the role that humanity plays in the game is what role the race has in the galaxy. For example, in ME1, humanity is just now becoming part of the council races, a group of aliens that have been traveling around the galaxy for a lot longer than humans have. As the game plays out, Shepard can do things that reflect a 'pro-unity' stance with the council races, or a 'independent/dominant' stance. This concept is put to the real test at the end when Shepard gets to decide on whether to save the members of the council or save the human ships. Do you work alongside with the other races, or do you step in and say you're the boss now?
That's one of the reasons I liked TIM as the antagonist in the second game. He represented the human element that doesn't want to be looked at as equals to all the other races in the galaxy. He wants humanity to be the 'dominate power' over everyone else and he was willing to do whatever it took to ensure that goal.
But in ME3, Earth and humanity are pretty much the priority no matter where your character stands.