Random Overdue Question on SubjectConfirmation

Can anyone explain the rational behind having the MUST NOT on the NotBefore attribute of <SubjectConfirmationData> in the Web Browerser SSO profile (section 4.1.4.2  of saml-core-2.0-os)?   It predates me and, though I didn't understand it, I didn't give it too much considereation when I wrote my SAML implementation.  However, now I'm looking at profiling the use of a SAML bearer assertion in a different but similar context and I'd like to better understand why the NotBefore was so explicitly disallowed so I can decide if I should follow suit or not. 

Thanks for any info,
Brian Campbell