To revoke a certificate you must keep a list of revoked certificates. Without expiration date that list would grow infinitely. And that list should be downloaded periodically by every entity which wants to verify certificate.
They said "certificates should never expire or should expire daily". Roots already can't be revoked, so they should never expire. Intermediates and leaves should expire daily. Since currently, OCSP responses are often valid for that long, there'd be no need for revocation anymore then.
