It's not a good idea. Many people will find this useful, but few will be annoyed. I guess you're thinking that pressing floor "2" once will light up the button, pressing again will cancel it and turn off the backlit. Here's a problem:
Elevators are used by everyone, old people, blind people, kids, conveying the information that the button for a particular floor is already pressed or not in a way that everyone understands in just a second isn't an easy task. Visually impaired who use braille on those buttons will not be able to tell. People not familiar with this backlit on off design may misunderstand it easily.
It'll be way too annoying if the lift doesn't stop where intended. A lift occasionally stopping at an extra floor is way better than a lift that occasionally skips the desired floor out of misunderstanding.