Sites like gmail and yahoo have a Remember Me checkbox. Cas could provide similar functionality.
A web site could contain three levels of protection:
1. Public - Any and all people can view this content ( Cas not used at all here ).
2. Previously authenticated - If you have been here before there is no need to login again. If you have the cookie, that is good enough.
3. Requires authentication. If the user want to change their password, the presence of the cookie is not good enough, they must enter user and password before they can access this level of functionality.
From the client side, it would be useful to know if they recently entered in credentials or if they accessed the site via cookie.