Might be a good move

> Unless there is absolutely no sanity left among mainstream Linux desktop programmers, there's no option to start a shell from those greeters. All you can do is click some stuff and enter text in the username and password fields.

Maybe the "greeter" is sane, but can the same be said for everything in its tech stack?

> limited to the ASCII keys you can physically type on the attached keyboard,

1) What about non-password-based login mechanisms?
2) What about non-ASCII (especially CJK that requires more advanced input methods)
3) _Which_ attached keyboard/mouse[/camera/biometric/etc]?

> Why do something ridiculous like using temporary user IDs for xdm?

1) "Defense in depth"
2) It's a lot less work than "rewrite everything else" (and inevitably introduce a different set of bugs/regressions)

> Most machines are only going to be running one copy of xdm anyway,

....And what about those which don't?