It is already the case. In the keynote, they demoed Fast User Switch with a touch of finger. If multiple people share a single computer, switching to the right user account is a touch away.

Yeah i saw that, but FUS needs a password to login, just like if you use the regular login screen.

I was talking about "X needs your password to Y", e.g. unlocking sys preferences panels, keychain stuff, etc.

... which opens up to the use case of: _____ ? of using a MBP as a cash register, where barmen can fastly display their list of customers ???

I haven't tried the new MBP but this was how I assumed it worked... On iOS you just need to enter your password when the device first boots, then you can use TouchID to authenticate for almost everything where you would use your password.

They've probably implemented TouchID support using PAM. If so, using with sudo/su is just a matter of configuration.

Seems unlikely to me, considering the auth is taking place on a separate SoC running WatchOS...