One effect of expiration dates is shorter revocation lists - if a revoked certificate was originally valid forever, the revocation list should list it since it

One effect of expiration dates is shorter revocation lists - if a revoked certificate was originally valid forever, the revocation list should list it since it was revoked to the end of times. Short lived certificates results in shorter CRLs. Lengthy CRLs have performance cost, they need to be downloaded and processed by clients. There is also a growing risk correlated to certificates validity length. If CRLs are unavailable to a client, the CRL server might be offline or unreachable or the client's access is maliciously blocked. The client have no knowledge of the reason and need to decide whether to trust the certificate without CRL check or not. If clients would be configured not to trust in such cases, the path to DOS is clear - block access to the CRL server and you effectively block traffic to all services of that CA. If clients are configured to trust certificates when CRLs are unavailable, which I beleive is the default on all OSs currently, blocking access to CRLs allows attacker to fool clients to trust revoked CRLs. With short living certificates, the opportunity window for such attacks is smaller.