Those saying you want Face ID on the Mac because you don't want to have to enter a password or use your Apple Watch to authenticate, there are still parts of macOS that require you to enter your password anyway. Like, on startup - have to enter your password before Touch ID or Unlock with Apple Watch will work; showing a password in Keychain Access; installing some software apps. Face ID won't end these requirements; it'll just be another means of authenticating with, maybe, some minor improvements over the existing methods.
For example:
- Touch ID on the MacBooks: You have to login with your password before you can use it to unlock it, and it's right there on the keyboard, in the top corner. You don't have to look for it.
- Unlock with Apple Watch: As above, you have to login before you can use it, but once you have it's just a matter of tapping a key or the Trackpad to make the Mac wake up and unlock.
- Authenticate with Apple Watch: A simple double-press of the Side Button authenticates most dialogs in macOS.
How would you choose NOT to authenticate a dialog or authenticate an Apple Pay payment in Safari if you have Face ID enabled? If you're looking at your screen and an authentication dialog pops up, does it immediately see your face and confirm the dialog? That's dangerous, so macOS might have to have a button on that dialog for you to press to authenticate with Face ID (as would any web page to confirm an Apple Pay payment).
Touch ID and Apple Watch require you to do something extremely simple to confirm you want to proceed. From what I've just theorised above, Face ID puts in an extra step.