I think knowing about µ฿ is a must if you are a bitcoiner. Being able to use bitcoin itself should mean that one should know know how it is paired and the units it can be measured with. But yeah I do agree that having the option of choosing the unit should be there as the users can select it as per their convenience while knowing the meaning of every unit is a must.
As I said, it brings more convenience for users, and it is a sort of personal choice and favorite. Anyway, such unit option is helpful for both sides (users and the platform). When users have more convenience and more comfortable experience, they will visit and use the platform more.
I agree with you. So people won't be confused and make an effort to search for the denomination again.
I was confused before about this unit because I don't know how much BTC it is and don't know what it's called.
I don't think people confuse about that, excluding newbies. I did not know what is µ฿ when I was newbie.

The option to setup default unit is definitely but it is not a must, a should instead.