What's the fun in that? I don't sing just to hum consistent tones until I finish the song. It's like asking why bother making any instruments more than zero dots because you can just get a bot to play it for you.
As far as actually tiering them sometimes you come across a song that takes more than just pitch to do well in. I would not consider a song like "Rap God" to be a zero dot song...if I'm actually singing, that is.
This is a topic I've thought about a lot. Not even just rap, all death metal is 0 dots, but can you actually sing like that? 
I feel like the difficulty of certain songs when you're trying to be authentic isn't done justice by the "talkies are 0 dots" rating system, HOWEVER, I think it's way too big a can of worms to get into the even more subjective debate about how to go about scoring them differently.
