one thing: language learning
Do we really want to dilute the uniqueness of language by making everyone sound like they came out of a lab in California?
Getting that wrong in some languages e.g. Korean can be offensive.
The people behind this demo already said their publishing different languages and accents soon along with open models you can run yourself.
I can't speak to whether it's desirable or not, but this has been happening with the advent of radio, movies, and television for over a century. So, are we worse off now, linguistically-speaking, than then? Do we really even notice missing accents if we never grew up with them?