If the authenticated user role does not have the 'post comments' permission, then the "Login or register to post comments" links are inappropriate, because the user may not be able to post comments even after logging in.
Users in that position might see this as a broken promise. This problem may be the reason for inventing the "you can't post comments" text (see http://drupal.org/node/137100), but that text doesn't solve the problem at all, on the contrary: it's a constant reminder of the broken promise and the fact that the user apparently belongs to a lesser user category.
I propose to remove "Login or register to post comments", if the authenticated user role does not have 'post comments' permission.
See also http://drupal.org/node/169938.