I'm certainly open to explanations as to what I'm missing; that is, why it's important to have this information in the certificate and what you'd do different because of it. But given that privateKeyUsagePeriod is permitted in a certificate for those who really want it (it's a SHOULD NOT, not a MUST NOT unless you want to mark it critical), and I personally don't see any benefit to it, I'm in favor of leaving the recommendation the way it is in 3280.