Why the heck does it matter if you can turn on or off every given item?  Are people seriously mad that they can't make a green shell/banana/bullet bill race?  Who cares?
90% of the customization people would do is already covered by the options they give.  You can play with them all on.  You can turn off all items, finally.  I could see just mushrooms being a popular choice too, and bananas only is a good non-overpowered choice.
I'm just not feeling any intense frustration at not being able to play a star/mushroom/lightning only race or whatever.