Well, you say that, and since Prime 3 is a sequel to Prime 2, which had a multiplayer, i can say, that the multiplayer on that, was Shit.
There is a lot of conflicting opinions regarding Metroid Prime Hunters Multiplayer, i absolutely hated it.
And yeah, adding multiplayer to a Metroid game, just makes the multiplayer another Generic Online FPS
Could Nintendo handle more then 4 players online, last i heard they were struggling on Smash Bros to do that.
For an online console FPS game, you need at least 8 players