Is this why you shouldn't log in as root?

[alex <radsky@ncia.net>]

Assume that you log in to Gnome as a user, call up a
terminal and then do su or sudo.

Does this give root access to Gnome or is root's
operation restricted to what it does in the
terminal while user can still operate in Gnome?

My thinking is that since user doesn't have the
permissions that root has, user can't do as much
damage in Gnome that root could.  So, by not logging
root in to Gnome (or KDE), root doesn't have the
opportunity to do any damage.

This could be true only if when you do su or sudo in
terminal, root's authority is restricted to that
terminal while user can still use Gnome.

Is this reasoning correct?