safe value for AuthenticationInstant?

i've been perusing the code of an open source implementation of 
saml 1.1's web sso profile to try and get a grasp on how saml's 
being implemented by other developers out there. here is a comment 
that appears in the code at the point where 
<AuthenticationStatement ... AuthenticationInstant="..." />  is 
set:

     "// No one seems to actually care about authn instant so
      // we'll just set it to [new java.util.Date()...]
      // until there are some other requirements..."

that struck me as a curious comment! i would think that the time a 
subject was authenticated is hugely important in most cases (to 
guard against replay, for example). how have developers in this 
forum used AuthenticationInstant in their projects?