I'll offer a possibly valid use case for Unicode identifiers: mathematical notation. C# allows Unicode for variable names, and I wrote some code using θ (theta) as a variable for a geometric angle. I really liked having a struct for cylindrical coordinates to name the values as (r, h, θ), which flowed quite well visually, better than writing out "theta" or "angle". And since it was a member of a struct, once I had the symbol in the editor once, the Intellisense code completion would just show the θ in the picker dropdown.
(It was for a personal project and I'm not sure I'd do that in a team environment, but I'd at least ask the other developers if they'd like it. And of course one use of Unicode being good doesn't mean that all uses of Unicode are good.)
Yes, this is the most common argument, but still, not in my back yard. I'm lucky enough to work with plenty of maths PhDs who don't insist on these shenanigans and still seem productive.
(It was for a personal project and I'm not sure I'd do that in a team environment, but I'd at least ask the other developers if they'd like it. And of course one use of Unicode being good doesn't mean that all uses of Unicode are good.)