The two reasons I can think of that vtys could me more useful are that
   a) you can have multiple login sessions (does window(1) let you do
   this?--I don't know)

I haven't thought carefully about the security implications, but I don't
see why having a script run on the console that sets it's SHELL to
/usr/bin/login and runs window(1) doesn't give you nearly the same
effect as virtual terminals (for text).  You could do something similar
in the X server and xdm.

The point is that all of this policy can *easily* be encapsulated in
user-level programs.  There's no need for it to be in the kernel.  The
only thing the kernel needs to be able to do is switch between text and
graphics modes, and have a terminal emulator.