Seriously, I'd hoped that Nintendo was wiser than to release a 4-player Mario game with only the brothers and two Toads playable. The series has more of a cast to offer than that, and it's just a wasted opportunity. You'd think the people who brought back the freaking Koopa Kids would have noticed that.
The basic thing is, they wanted to make sure that each player was playing with the same "playstyle", and had a similar model to match. Everybody in the game plays just like Mario. Luigi doesn't have his "floaty" jumps, Toad doesn't run any faster after picking items up, or any other frivolous thing.
In short, the other option would have been to just have as play as 4 different colored Marios, and we would have gotten the same result (and likely the same type of complaints
).
Frankly, the only thing I question is why it had to be two toads, when one of them could have been something like a Hammer(less) Bro. It could have also sufficed. I mean, thanks to the Paper Mario games, especially, we know that some Troopas have no problem with co-oping with Mario and da crew. Plus, a Hammer Bro would have been close to Mario's build, and could have been modeled to follow suit with the same play style.
Such a thing would not have put a stop to the various belly-aching out there, but I think it would have at least given a bit more variety than a straight clone of the 3P.