The reviews complain about the multiplayer seeming tacked on, but the only other ways would be split screen or turn based. Turn based would be cool, but would make us wait longer. Split Screen would be hard to work in because getting certain items or getting to certain areas trigger events that could block the path of player 2.
At least we have some sort of multiplayer, and at least we get to see Luigi this time.
Then again, we lost Yoshi and Luigi isn't exactly ride-able.