In NSMB Wii, you COULD play two players, kill off Mario, and play as another character for the entire game. Only problem is that every 100 coins, you get an extra Life for Mario and his bubble floats around. Feel free to kill him again though. Kind of a roundabout way to do it, and you still have to use P1's controller on the map screen, but if you really want to play as Luigi or a Toad in the standard game, it's a good way to do it.
RIP Hero Fly Chao http://badges.mypersonality.info/badge/0/23/238561.png New posters, read: http://pbskids.org/arthur/games/factsopinions/
Seems like an oversight by the developers. If someone has 5 stars, they obviously gain no advantage due to Luigi's height or whatever so they should let you play as him. It takes no effort to add this functionality.