Onyxmeth,
No one is going to try their hardest and still need to use Luigi in every single level.
If people get to see more levels, with different traps and enemies, and different environments and music, they'll keep playing more, and keep improving. If you have to keep playing the same thing over and over again until you beat it, you don't necesarily get better at the rest of the game.
I remember being stuck for MONTHS on King Zing Sting in Donkey Kong Country 2, when I was young. And after beating it, there was no other level similar to it afterwards. So it didn't help me improve, it was just frustrating.
Now as an adult, with (at least a little) less free time, I don't know if I would go back and beat such a level if I were able to skip it. But does that matter? The levels after King Zing Sting are amazing, and why shouldn't I be able to have fun with those?
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.